The Gap Junction Communication Channel.

Communication plays an important role in the rapid progress of modern society. We live in an age when information is transmitted through many different pathways and in many different forms to influence our daily decisions. However, although we live in communities, we still treasure our individuality. By analogy, the survival of multicellular organisms depends on each cell type retaining its individuality, even though all cellular activities must be coordinated with other cells. Organisms have evolved multiple strategies to achieve this goal, which include long-range interactions mediated by neural or endocrine mechanisms or short-range interactions that include direct physical or cell–cell contact. While the first strategy involves interactions at a distance, one mode of direct communication involves the cell-to-cell transmission of molecules through channels in a specialized cell surface membrane structure, the gap junction. Gap junctions contain channels that connect neighboring cells. They differ from other membrane channels since they exist between two cells, they are relatively nonspecific, and the molecular movement through the channels occurs by passive diffusion. The gap junctional channels have an apparent selectivity based principally on molecular size, allowing the movement of molecules smaller than 1000 Da, such as cAMP, but preventing the movement of proteins or nucleic acids. Small informational molecules, such as certain morphogens, could be directly transmitted between cells via gap junctions. Consequently, this type of communication is an important mechanism for regulating events between cells during embryogenesis (56Warner A.E Guthrie S.C Gilula N.B Antibodies to gap junctional protein selectively disrupt junctional communication in the early amphibian embryo.Nature. 1984; 311: 127-131Crossref PubMed Scopus (290) Google Scholar) and during the normal function of organs, such as the heart. Apart from a few terminally differentiated cells, such as skeletal muscle, erythrocytes, and circulating lymphocytes, most cells in normal tissues generally communicate via gap junctions. These junctions exist in almost all animals, both vertebrates and invertebrates, and higher plant cells utilize a similar mechanism for cell–cell communication via the plasmodesmata structures. There has been rapid progress recently in identifying and characterizing a multigene family that codes the gap junction proteins (called connexins). Two alternative nomenclature systems, one based on the molecular mass of the connexin polypeptide and the other based on evolutionary considerations (18Kumar N.M Gilula N.B Molecular biology and genetics of gap junction channels.Semin. Cell Biol. 1992; 3: 3-16Crossref PubMed Scopus (188) Google Scholar), are currently in use. In this review, the Greek nomenclature system will be used. The relationship between the two nomenclature systems is shown in Table 1.Table 1Connexin Multigene FamilyGreek Letter NomenclatureMolecular Mass NomenclaturePredicted Molecular Mass (kDa)Examples of Organs with Expressionα1Cx4343.0Heartα2Cx3837.8Embryoα3Cx4646.0Lensα4Cx3737.6Lungα5Cx4040.4Lungα6Cx4545.7Heartα7Cx3332.9Testisα8Cx5049.6Lensβ1Cx3232.0Liverβ2Cx2626.5Liverβ3Cx3131.0Skinβ4Cx31.131.1Skinβ5Cx30.330.3SkinDeduced protein size to nearest 0.1 kDa predicted from rodent connexin cDNA analysis is shown except for α2 connexin, in which Xenopus cDNA was analyzed. Owing to space limitations, proper citations are not included. The reader is referred to the GenBank database for proper citations and to 18Kumar N.M Gilula N.B Molecular biology and genetics of gap junction channels.Semin. Cell Biol. 1992; 3: 3-16Crossref PubMed Scopus (188) Google Scholar, 47Spray D.C Dermietzel R X-linked dominant Charcot–Marie–Tooth disease and other potential gap junction disease of the nervous system.Trends Neurosci. 1995; 18: 256-262PubMed Scopus (90) Google Scholar, 31Musil, L.S. (1994). Structure and assembly of gap junctions. In Molecular Mechanisms of Epithelial Cell Junctions: From Development to Disease, S. Citi, ed. (Austin, Texas: R.G. Landes Company), pp. 173–194.Google Scholar. Open table in a new tab Deduced protein size to nearest 0.1 kDa predicted from rodent connexin cDNA analysis is shown except for α2 connexin, in which Xenopus cDNA was analyzed. Owing to space limitations, proper citations are not included. The reader is referred to the GenBank database for proper citations and to 18Kumar N.M Gilula N.B Molecular biology and genetics of gap junction channels.Semin. Cell Biol. 1992; 3: 3-16Crossref PubMed Scopus (188) Google Scholar, 47Spray D.C Dermietzel R X-linked dominant Charcot–Marie–Tooth disease and other potential gap junction disease of the nervous system.Trends Neurosci. 1995; 18: 256-262PubMed Scopus (90) Google Scholar, 31Musil, L.S. (1994). Structure and assembly of gap junctions. In Molecular Mechanisms of Epithelial Cell Junctions: From Development to Disease, S. Citi, ed. (Austin, Texas: R.G. Landes Company), pp. 173–194.Google Scholar. This review will highlight recent progress in understanding the biosynthesis, assembly, and structure of the vertebrate gap junction channel, as well as its relationship to several human diseases. Owing to a space limitation, it is not possible to provide a comprehensive review of the progress in the gap junction field. However, there are several recent reviews that can be used to obtain more information about other connexin-related topics (31Musil, L.S. (1994). Structure and assembly of gap junctions. In Molecular Mechanisms of Epithelial Cell Junctions: From Development to Disease, S. Citi, ed. (Austin, Texas: R.G. Landes Company), pp. 173–194.Google Scholar, 21Leclerc, C. (1994). The roles of gap junctions in early development. In Molecular Mechanisms of Epithelial Cell Junctions: From Development to Disease, S. Citi, ed. (Austin, Texas: R.G. Landes Company), pp. 45–65.Google Scholar, 46Spray, D.C. (1994). Physiological and pharamacological regulation of gap junction channels. In Molecular Mechanisms of Epithelial Cell Junctions: From Development to Disease, S. Citi, ed. (Austin, Texas: R.G. Landes Company), pp. 195–215.Google Scholar, 15Kanno, Y., Kataoka, K., Shiba, Y., Shibata, Y., and Shimazu, T., eds. (1995). Progress in Cell Research, Volume 4: Intercellular Communication through Gap Junctions (Amsterdam: Elsevier Sciences).Google Scholar, 59Wolburg H Rohlmann A Structure–function relationships in gap junctions.Int. Rev. Cytol. 1995; 157: 315-373Crossref PubMed Scopus (104) Google Scholar). Gap junctions exhibit a hierarchy of assembly. The principal structural component, the membrane protein connexin, is organized into the basic unit of structure, the connexon, which is a hexameric structure with a toroid appearance in negative-stained preparations. The family of connexin proteins includes at least 12 members in rodents (Table 1; 18Kumar N.M Gilula N.B Molecular biology and genetics of gap junction channels.Semin. Cell Biol. 1992; 3: 3-16Crossref PubMed Scopus (188) Google Scholar). An individual connexon from one cell docks or associates with a corresponding connexon on a neighboring cell to form a gap junction channel, and multiple channels, in turn, cluster or aggregate in the plane of the membrane to form gap junction plaques. The properties of the gap junction channels are defined by the connexins. Structural and biophysical studies are being used to define the mechanism by which connexins function. Connexins are the principal protein component of gap junctions. There is much evidence to support the fact that the connexins alone (assembled in a lipid bilayer) are responsible for the generation of gap junctional channels. This evidence includes the following: connexin sequences are consistent with an integral membrane protein that has a transmembrane domain containing polar amino acids that would contribute to the formation of a channel lining; reconstitution of purified connexins into artificial membranes yields functional channels (reviewed by6Buehler L.K Stauffer K.A Gilula N.B Kumar N.M Single channel behavior of recombinant β2 gap junction connexons reconstituted into planar lipid bilayers.Biophy. J. 1995; 68: 1767-1775Abstract Full Text PDF PubMed Scopus (39) Google Scholar); expression of connexin cDNAs in heterologous systems (including yeast) yields not only functional gap junction channels, but also gap junctions that are ultrastructurally identical to those occurring naturally in vivo; electron microscopic immunocytochemical studies localize connexins to gap junction plaques; and the distribution of connexins observed in vivo can be related to gap junctional communication pathways. Each of the connexins appears to fit the general topological model for gap junction protein (Figure 1). In this model, the polypeptide traverses the lipid bilayer four times, with both the N- and C-termini facing the cytoplasm (29Milks L.C Kumar N.M Houghten N Unwin N Gilula N.B Topology of the 32-kD liver gap junction protein determined by site-directed antibody localizations.EMBO J. 1988; 7: 2967-2975Crossref PubMed Scopus (253) Google Scholar; cf.60Yeager M Gilula N.B Membrane topology and quaternary structure of cardiac gap junction ion channels.J. Mol. Biol. 1992; 223: 929-948Crossref PubMed Scopus (115) Google Scholar). Analysis of the different connexins indicates that one of the transmembrane domains, M3, has an amphipathic character, suggesting that it contributes to the lining of the channel. The two extracellular loops (E1 and E2) are thought to be involved in initiating the interaction between connexons in adjacent cells. A set of three cysteine residues exists in each of the extracellular loops with a characteristic arrangement that is a signature of connexins. These may help to maintain the rigid tertiary structure that enables two opposing connexons to dock with each other. The regions between the transmembrane domains M2 and M3, as well as the C-termini of the connexins, are highly variable among the different connexins and are, therefore, thought to be important for regulation. It has been suggested that the folding pattern for the connexins corresponds to an antiparallel arrangement of four transmembrane domains that associate to form a left-handed bundle (29Milks L.C Kumar N.M Houghten N Unwin N Gilula N.B Topology of the 32-kD liver gap junction protein determined by site-directed antibody localizations.EMBO J. 1988; 7: 2967-2975Crossref PubMed Scopus (253) Google Scholar), which is consistent with the known structural and permeability properties of gap junctions. X-ray (53Tibbitts T.T Caspar D.L.D Phillips W.C Goodenough D.A Diffraction diagnosis of protein folding in gap junction connexons.Biophys. J. 1990; 57: 1025-1036Abstract Full Text PDF PubMed Scopus (31) Google Scholar) and circular dichroism studies (7Cascio M Kumar N.M Safarik R Gilula N.B Physical characterization of gap junction membrane connexons (hemi-channels) isolated from rat liver.J. Biol. Chem. 1995; 270: 18643-18648Crossref PubMed Scopus (49) Google Scholar) are consistent with the high helical content of the transmembrane domains of β1 connexin predicted by this model. However, it should be stressed that such models are speculative since there is little relevant evidence available. Much progress has yet to be made in obtaining some structural information on the gap junction connexins at the atomic level. The oligomeric arrangement of connexins has been indicated in structural studies on gap junctions where 6-fold symmetry has been used as a constraint in the image analysis. Recently, independent evidence has been provided by chemical cross-linking studies on purified rat liver gap junction connexons (7Cascio M Kumar N.M Safarik R Gilula N.B Physical characterization of gap junction membrane connexons (hemi-channels) isolated from rat liver.J. Biol. Chem. 1995; 270: 18643-18648Crossref PubMed Scopus (49) Google Scholar) to indicate that each connexon consists of six subunits. Although the stoichiometry for the association of connexins in other connexons is not yet known, it would be reasonable, based on their similar predicted topology, to expect that the other connexins will also associate to form hexameric connexons. In addition to inorganic ions (Na+, K+, Ca2+, etc.), a number of small molecules, such as cAMP and inositol 1,4,5-trisphosphate (IP3), can pass through gap junctions (reviewed by25Loewenstein W.R Junctional intercellular communication the cell-to-cell membrane channel.Physiol. Rev. 1981; 61: 829-913Crossref PubMed Scopus (896) Google Scholar, 46Spray, D.C. (1994). Physiological and pharamacological regulation of gap junction channels. In Molecular Mechanisms of Epithelial Cell Junctions: From Development to Disease, S. Citi, ed. (Austin, Texas: R.G. Landes Company), pp. 195–215.Google Scholar). Using peptides conjugated to fluorescent dyes, researchers have estimated the size of the pore for a gap junction channel (25Loewenstein W.R Junctional intercellular communication the cell-to-cell membrane channel.Physiol. Rev. 1981; 61: 829-913Crossref PubMed Scopus (896) Google Scholar). This analysis indicates that gap junctions are relatively nonselective in their permeability to hydrophilic molecules with diameters of 1.5 nm or less. Since the earlier permeability studies, it has become apparent that the gap junction channels can be formed by a number of different connexins. Therefore, it is important to consider the fact that the channels formed by different connexins may have different permeabilities that will permit the discrimination of second messengers, such as cAMP, cGMP, Ca2+, or IP3. The net result of such diversity could be to form communication compartments that will enable a select group of cells to be regulated by changes in the concentration of a specific second messenger or metabolite. Recently, it has been demonstrated that there are connexin-specific differences in channel permeabilities (2Brissette J.L Kumar N.M Gilula N.B Hall J.E Dotto G.P Switch in gap junction protein expression is associated with selective changes in junctional permeability during keratinocyte differentiation.Proc. Natl. Acad. Sci. USA. 1994; 91: 6453-6457Crossref PubMed Scopus (123) Google Scholar, 51Steinberg T.H Civitelli R Geist S.T Roberston A.J Hick E Veenstra R.D Wang H.-Z Warlow P.M Westphale E.M Laing J.G Beyer E.C Connexin43 and connexin45 form gap junctions with different molecular permeabilities in osteoblastic cells.EMBO J. 1994; 13: 744-750Crossref PubMed Scopus (221) Google Scholar, 9Elfgang C Eckert R Lichtenberg-Fraté H Butterweck A Traub O Klein R.A Hülser D.F Willecke K Specific permeability and selective formation of gap junction channels in connexin-transfected HeLa cells.J. Cell Biol. 1995; 129: 805-817Crossref PubMed Scopus (721) Google Scholar, 55Veenstra R.D Wang H.-Z Beblo D.A Chilton M.G Harris A.L Beyer E.C Brink P.R Selectivity of connexin-specific gap junctions does not correlate with channel conductance.Circ. Res. 1995; 77: 1156-1165Crossref PubMed Scopus (216) Google Scholar). These channel permeabilities are either intrinsic to the structure of the connexin or due to differences in posttranslational modifications. Differences in channel permeabilities may be important for normal physiological processes since a modulation in expression of different connexin isoforms has been observed in many systems. For example, during keratinocyte differentiation, there is a switch in expression from α1 and β2 connexin to β3 and β4 connexin (2Brissette J.L Kumar N.M Gilula N.B Hall J.E Dotto G.P Switch in gap junction protein expression is associated with selective changes in junctional permeability during keratinocyte differentiation.Proc. Natl. Acad. Sci. USA. 1994; 91: 6453-6457Crossref PubMed Scopus (123) Google Scholar). Correlated with these changes in connexin expression are changes in junctional permeability as indicated by a decreased transfer of large molecules, such as the dye lucifer yellow, even though the cells remain electrically coupled (2Brissette J.L Kumar N.M Gilula N.B Hall J.E Dotto G.P Switch in gap junction protein expression is associated with selective changes in junctional permeability during keratinocyte differentiation.Proc. Natl. Acad. Sci. USA. 1994; 91: 6453-6457Crossref PubMed Scopus (123) Google Scholar). It remains to be determined whether different biologically relevant molecules pass between the keratinocytes at different stages of differentiation. Interestingly, the transfer of metabolites such as amino acids and nucleotides did not appear to be influenced by the change in connexin expression. Thus, it is unlikely that size is the only consideration for distinguishing the permeability properties of connexins. In a recent study, it was demonstrated that the relative selectivity and permeability to ions and dyes for different connexins was independent of channel conductance (55Veenstra R.D Wang H.-Z Beblo D.A Chilton M.G Harris A.L Beyer E.C Brink P.R Selectivity of connexin-specific gap junctions does not correlate with channel conductance.Circ. Res. 1995; 77: 1156-1165Crossref PubMed Scopus (216) Google Scholar). The results in this study imply that weak electrostatic potentials associated with the pore of each connexin channel account for the gap junction channel conductance and permselectivity. However, the molecular basis for the apparent selectivity in channel permeability is not known. Reconstitution studies and the biophysical characterization of gap junction channel properties between cells have provided indications that different connexins have single channel conductances ranging from 20 pS to several hundred picoSiemens (46Spray, D.C. (1994). Physiological and pharamacological regulation of gap junction channels. In Molecular Mechanisms of Epithelial Cell Junctions: From Development to Disease, S. Citi, ed. (Austin, Texas: R.G. Landes Company), pp. 195–215.Google Scholar). The different single channel conductances may reflect intrinsic differences in the sequences of the connexins. For example, some connexins are known to be phosphorylated by tyrosine kinases (31Musil, L.S. (1994). Structure and assembly of gap junctions. In Molecular Mechanisms of Epithelial Cell Junctions: From Development to Disease, S. Citi, ed. (Austin, Texas: R.G. Landes Company), pp. 173–194.Google Scholar). These phosphorylation events, as well as other posttranslational modifications, such as palmitoylation, may induce conformational changes that influence conductance and permeability properties of the connexins. Thus, gap junction channels, like other channels, may be regulated by gating, and this may enable a cell to respond rapidly to various physiological events. Since many ion channels are voltage dependent, it is perhaps not surprising that gap junction channels are also voltage dependent, although the magnitude of the dependency varies with the type of connexin (46Spray, D.C. (1994). Physiological and pharamacological regulation of gap junction channels. In Molecular Mechanisms of Epithelial Cell Junctions: From Development to Disease, S. Citi, ed. (Austin, Texas: R.G. Landes Company), pp. 195–215.Google Scholar). It should, however, be noted that the biological relevance of this dependency on connexin function remains to be elucidated. Interestingly, the connexins lack the characteristic arrangement of charged amino acids that is typical of other voltage-gated channels, such as Na+ and Ca2+ channels. The voltage sensitivity of ion channels is thought to involve the movement of electrical charges in the protein in response to the potential difference across the membrane; this would require a conformational change in the ion channel protein. This has been envisioned either as a physical blockage (ball on a chain model) or as a rotation and tilting of the connexins (iris model) resulting in an alteration in pore size. Ca2+ was the first cytoplasmic factor to be implicated in the regulation of gap junction function (25Loewenstein W.R Junctional intercellular communication the cell-to-cell membrane channel.Physiol. Rev. 1981; 61: 829-913Crossref PubMed Scopus (896) Google Scholar). Ca2+ may play a dual role in both gating of the channel, as well as being transferred by diffusion through the channel. While an increase in Ca2+ concentration has been correlated with reduced gap junctional communication, this occurs at nonphysiological Ca2+ concentrations (46Spray, D.C. (1994). Physiological and pharamacological regulation of gap junction channels. In Molecular Mechanisms of Epithelial Cell Junctions: From Development to Disease, S. Citi, ed. (Austin, Texas: R.G. Landes Company), pp. 195–215.Google Scholar). However, a gating of gap junctions by changes in intracellular Ca2+ may be important under pathological conditions. More recently, there have been numerous indications that oscillatory changes in Ca2+ concentration and cell–cell propagating waves of free Ca2+ at physiologically relevant concentrations, perhaps mediated via the cell–cell movement of IP3 (44Sáez J.C Connor J.A Spray D.C Bennett M.V.L Hepatocyte gap junctions are permeable to the second messenger, inositol 1,4,5-triphosphate, and to calcium ions.Proc. Natl. Acad. Sci. USA. 1989; 86: 2708-2712Crossref PubMed Scopus (492) Google Scholar, 45Sanderson M.J Charles A.C Boitano S Dirksen E.R Mechanisms and function of intercellular calcium signaling.Mol. Cell Endocrinol. 1994; 98: 173-187Crossref PubMed Scopus (265) Google Scholar), may serve to coordinate important activities in various physiological conditions, including neuron–glial cell interactions. Thus, there are indications that changes in intracellular Ca2+ in glial cells can cause changes in intracellular Ca2+ levels of surrounding neuronal cells and that this process is mediated by gap junctions between the two cell types (35Nedergaard M Direct signaling from astrocytes to neurons in cultures of mammalian brain cells.Science. 1994; 263: 1768-1771Crossref PubMed Scopus (819) Google Scholar). However, one potential difficulty with this interpretation is that astrocytes express α1 connexin, whereas neurons express β1 connexin (8Dermietzel R Spray D.C Gap junctions in the brain where, what type, how many and why.Trends Neurosci. 1993; 16: 186-192Abstract Full Text PDF PubMed Scopus (440) Google Scholar). These two connexin isoforms have been reported to be incompatible in the paired Xenopus oocyte system (58White T.W Paul D.L Goodenough D.A Bruzzone R Functional analysis of selective interactions among rodent connexins.Mol. Biol. Cell. 1995; 6: 459-470Crossref PubMed Scopus (216) Google Scholar). Consequently, if the oocyte data is relevant, then it remains to be determined whether other, as yet unidentified, connexins that are compatible with each other are also expressed in these two cell types. Alternatively, another mechanism, such as a glutamate-gated channel, may be involved in the transmission of Ca2+ activation signals from astrocytes to neurons (37Parpura V Basarsky T.A Liu F Jeftinija K Jeftinija S Haydon P.G Glutamate mediated astrocyte neuron signalling.Nature. 1994; 369: 744-747Crossref PubMed Scopus (1360) Google Scholar). The different connexin isoforms may associate with each other in many different combinations. This could significantly influence properties such as permeability and gating of the gap junction channels that are formed. Theoretically, a gap junction channel between two cells may contain two identical connexons containing the same type of connexin (a homotypic gap junction), or it may contain a different connexon composed of a different connexin or different stoichiometry in each half of the cell pair (heterotypic gap junction) (Figure 2). Furthermore, the potential incompatibility between certain pairs of connexons may have other consequences, such as the formation of communication barriers between groups of cells in contact. For example, since α5 connexin is expressed in Purkinje fibers and predominately α1 connexin is expressed in the ventricular myocardium, it has been suggested that the incompatibility between α1 and α5 prevents the coupling of these two cell types, which may contribute to the unique electrical characteristics of these two regions (4Bruzzone R Haefliger J.-A Gimlich R.L Paul D.L Connexin40, a component of gap junctions in vascular endothelium, is restricted in its ability to interact with other connexins.Mol. Biol. Cell. 1993; 4: 7-20Crossref PubMed Scopus (258) Google Scholar). The selective association of connexins may provide an explanation for the diversity in gap junction genes and the reason certain connexins are utilized by specific cell types. The results from model systems indicate that not all connexons will pair with each other, even though connexins are highly homologous to each other, especially in their extracellular domains (4Bruzzone R Haefliger J.-A Gimlich R.L Paul D.L Connexin40, a component of gap junctions in vascular endothelium, is restricted in its ability to interact with other connexins.Mol. Biol. Cell. 1993; 4: 7-20Crossref PubMed Scopus (258) Google Scholar, 9Elfgang C Eckert R Lichtenberg-Fraté H Butterweck A Traub O Klein R.A Hülser D.F Willecke K Specific permeability and selective formation of gap junction channels in connexin-transfected HeLa cells.J. Cell Biol. 1995; 129: 805-817Crossref PubMed Scopus (721) Google Scholar, 58White T.W Paul D.L Goodenough D.A Bruzzone R Functional analysis of selective interactions among rodent connexins.Mol. Biol. Cell. 1995; 6: 459-470Crossref PubMed Scopus (216) Google Scholar). Such studies suggest that the second extracellular domain (E2) is important for compatibility between connexins (57White T.W Bruzzone R Wolfram S Paul D.L Goodenough D.A Selective interactions among the multiple connexin proteins expressed in the vertebrate lens the second extracellular domain is a determinant of compatibility between connexins.J. Cell Biol. 1994; 125: 879-892Crossref PubMed Scopus (218) Google Scholar, 58White T.W Paul D.L Goodenough D.A Bruzzone R Functional analysis of selective interactions among rodent connexins.Mol. Biol. Cell. 1995; 6: 459-470Crossref PubMed Scopus (216) Google Scholar). In spite of the intriguing results from the studies cited above, there is no direct evidence for the existence of heterotypic gap junctions in vivo. Nonetheless, studies of such channels in heterologous cell systems have provided valuable perspective on the potential mechanisms that may regulate the formation and function of gap junction channels in vivo. Most cells express more than one type of connexin, leading to the possibility that heteromeric connexons (composed of two or even more connexins) may exist in vivo. In principal, connexons can consist of only one type (homomeric connexons) or multiple connexins (heteromeric connexons) (Figure 2). Since different connexins have different permeability properties, it is reasonable to assume that connexons that contain multiple connexins (heteromeric connexons) will have unique physiological properties. Thus, the specific communication pathways that exist between cells are likely to be influenced by the utilization of different connexins. The existence of heteromeric connexons has been supported recently by biochemical experiments that have utilized fractionation of detergent solubilized gap junctions (50Stauffer K.A Kumar N.M Gilula N.B Unwin N Isolation and purification of gap junction channels.J. Cell Biol. 1991; 115: 141-150Crossref PubMed Scopus (95) Google Scholar, 16Kistler J Bond J Donaldson P Engel A Two distinct levels of gap junction assembly in vitro.J. Struct. Biol. 1993; 110: 28-38Crossref PubMed Scopus (14) Google Scholar, 7Cascio M Kumar N.M Safarik R Gilula N.B Physical characterization of gap junction membrane connexons (hemi-channels) isolated from rat liver.J. Biol. Chem. 1995; 270: 18643-18648Crossref PubMed Scopus (49) Google Scholar, 49Stauffer K.A The gap junction proteins β1-connexin (connexin-32) and β2-connexin (connexin-26) can form heteromeric hemichannels.J. Biol. Chem. 1995; 270: 6768-6772Abstract Full Text Full Text PDF PubMed Scopus (60) Google Scholar). Some of these data suggest indirectly that β1 and β2 connexin can oligomerize to form a heteromeric connexon. More direct experimental approaches must be used in the near future to establish the existence of heteromeric connexons, as well as to determine whether there is selectivity in the oligomerization process. Communication compartments have been described in the mouse embryo (24Lo C.W Gilula N.B Gap junctional communication in the postimplantation mouse embryo.Cell. 1979; 18: 411-422Abstract Full Text PDF PubMed Scopus (154) Google Scholar) and in the mouse epidermis (14Kam E Melville L Pitts J.D Patterns of junctional communication in skin.J. Invest. Dermatol. 1986; 87: 748-753Abstract Full Text PDF PubMed Google Scholar). Such compartments may reflect the differential distribution of connexins within the plasma membrane of keratinocytes (11Goliger J.A Paul D.L Expression of gap junction proteins Cx26, Cx31.1, Cx37, and Cx43 in developing and mature rat epidermis.Dev. Dyn. 1994; 200: 1-13Crossref PubMed Scopus (133) Google Scholar). Although the molecular mechanism(s) for the selective targeting of connexins is not known, recent experiments using chimeras of synaptophysin and connexin indicate that the four transmembrane domains are necessary for proper targeting to the plasma membrane (23Leube R.E The topogenic fate of the polytopic transmembrane proteins, synaptophysin and connexin, is determined by their membrane-spanning domains.J. Cell Sci. 1995; 108: 883-894PubMed Google Scholar). The communication pathway mediated by gap junctions occurs between two cells, and it can usually be recognized by the presence of a gap junctional