The subunits in calpain and in the related penta‐EF‐hand (PEF) proteins are bound through contacts between the unpaired EF‐hand 5 from each subunit. To study subunit binding further, a tetra‐EF‐hand 18 kDa N‐ and C‐terminally truncated form of the calpain small subunit was prepared (18k). This protein does not combine with the calpain large subunit to form active calpain, but forms homodimers in solution, as shown by ultracentrifugation. The X‐ray structure of the 18k protein in the presence of cadmium was solved to a resolution of 2.0 Å. The structure of the monomer is almost identical to the known structure of the calpain small subunit, but the 18k protein forms an oligomer in the crystal by the use of two binding sites. One of these sites is an artefact arising from the C‐terminal truncation, but the other is a naturally occurring site that is fully exposed to water in intact purified calpain. The characteristics of this site suggest that it may be important in binding other protein modulators involved in the regulation of calpain and of PEF proteins. Proteins 2003. © 2003 Wiley‐Liss, Inc.

Production and refolding of recombinant Choristoneura fumiferana antifreeze protein (CfAFP) leads to a disulfide-bonded product containing dynamic conformational microheterogeneity. Difficulties in the crystallization of this protein arising from its microheterogeneity were overcome by screening of crystallization conditions at various temperatures and finally using a temperature of 318 K to obtain diffraction-quality crystals. In addition, heavy-atom derivatization of this protein required the iodination of a specific tyrosine residue, leading to the successful single anomalous scattering (SAS) structure determination. The techniques of higher temperature screening, to reduce dynamic conformational microheterogeneity, and defined tyrosine iodination, for specific heavy-atom incorporation, are methods which can be employed with other proteins to aid in structure determination.

The insect spruce budworm (Choristoneura fumiferana)(Cf) produces a number of isoforms of its highly active antifreeze protein (CfAFP). Although most of the CfAFP isoforms are in the 9-kDa range, isoforms containing a 30- or 31-amino acid insertion have also been identified. Here we describe the functional and structural analysis of a selected long isoform, CfAFP-501. X-ray crystal structure determination reveals that the 31-amino acid insertion found in CfAFP-501 forms two additional loops within its highly regular beta-helical structure. This effectively extends the area of the two-dimensional Thr array and ice-binding surface of the protein. The larger isoform has 3 times the thermal hysteresis activity of the 9-kDa CfAFP-337. As well, a deletion of the 31-amino acid insertion within CfAFP-501 to form CfAFP-501-Delta-2-loop, results in a protein with reduced activity similar to the shorter CfAFP isoforms. Thus, the enhanced antifreeze activity of CfAFP-501 is directly correlated to the length of its beta-helical structure and hence the size of its ice-binding face.

Reported here is the 2.3 Å resolution crystal structure of spruce budworm (Choristoneura fumiferana) antifreeze protein (CfAFP), solved by single anomalous scattering. The structure reveals an extremely regular left-handed β-helical platform consisting of 15-amino acid loops with a repetitive Thr-X-Thr motif displayed on one of the helix's three faces. This motif results in a two-dimensional array of threonine residues in an identical orientation to those in the nonhomologous, right-handed β-helical beetle AFP from Tenebrio molitor (TmAFP). The CfAFP structure led us to reevaluate our ice binding model, and the analysis of three possible modes of docking gives rise to a binding mechanism based on surface complementarity. This general mechanism is applicable to both fish and insect AFPs.