Myosin XXI is the only myosin expressed in Leishmania parasites. Although it is assumed that it performs a variety of motile functions, the motor's oligomerization states, cargo-binding, and motility are unknown. Here we show that binding of a single calmodulin causes the motor to adopt a monomeric state and to move actin filaments. In the absence of calmodulin, nonmotile dimers that cross-linked actin filaments were formed. Unexpectedly, structural analysis revealed that the dimerization domains include the calmodulin-binding neck region, essential for the generation of force and movement in myosins. Furthermore, monomeric myosin XXI bound to mixed liposomes, whereas the dimers did not. Lipid-binding sections overlapped with the dimerization domains, but also included a phox-homology domain in the converter region. We propose a mechanism of myosin regulation where dimerization, motility, and lipid binding are regulated by calmodulin. Although myosin-XXI dimers might act as nonmotile actin cross-linkers, the calmodulin-binding monomers might transport lipid cargo in the parasite.

Myosin-XXI is one of only two myosins found in the Leishmania parasite genome. While no expression of myosin-IB has been found in the organism to date, myosin-XXI has been detected in both the amastigote and promastigote stages of the Leishmania life cycle. The presence of only a single myosin isoform suggests that this myosin carries out a variety of functions within the protozoa, including membrane anchorage, longer range directed movements of cargo and possibly roles in cell signalling. Our aim is to investigate how a single myosin can carry out several different tasks within the cell and to identify molecular mechanisms controlling this. To determine the directionality of motor movement we performed gliding filament assays using myosin-XXI constructs expressed using a baculovirus/SF21 system and dual labelled F-actin with actin-filaments capped by gelsolin and labelled with phalloidin-TRITC at their barbed ends and phalloidin-FITC at their pointed ends. These experiments showed that myosin-XXI is a plus-end directed motor. Our in vitro studies also showed that myosin-XXI binds to a variety of lipids including PIP2 and PIP3 as well as a number of other phospholipids. Furthermore, the motor can adopt both a monomeric and a dimeric conformation in vitro. using a variety of tail constructs we found that only the monomeric conformation has the ability to bind lipids. We identified several distinct lipid-binding sites in the tail domain with different lipid binding specificities. Preliminary data suggest that motor dimerisation and lipid binding are regulated by binding of calcium-calmodulin which might play a key role in the cellular distribution of the motor and its ability to perform a variety of motile roles within the parasite. Sponsored by DFG-SFB 863 and Baur-Stiftung.

We present the photovoltaic application of a donor-acceptor system consisting of very large polycyclic aromatic hydrocarbons. Using vacuum sublimated hexa-peri-hexabenzocor-onenes we evidence long exciton diffusion lengths of approximately 25 nm. In conjunction with the heaxfluorinated analogue for the first time a photovoltaic device using nanographene as active material for both donor and acceptor compounds was fabricated. The bi-layered device exhibits a remarkably high open circuit voltage of 1.39 V. Light absorption of the photoactive materials used here is strictly confined to wavelengths below 500 nm, rendering this approach especially interesting for the application in semi-transparent devices as well as multi-layered and tandem solar cells.

Self-assembled monolayers of phosphonates (SAMPs) of 11-hydroxyundecylphosphonic acid, 2,6-diphosphonoanthracene, 9,10-diphenyl-2,6-diphosphonoanthracene, and 10,10'-diphosphono-9,9'-bianthracene and a novel self-assembled organophosphonate duplex ensemble were synthesized on nanometer-thick SiO(2)-coated, highly doped silicon electrodes. The duplex ensemble was synthesized by first treating the SAMP prepared from an aromatic diphosphonic acid to form a titanium complex-terminated one; this was followed by addition of a second equivalent of the aromatic diphosphonic acid. SAMP homogeneity, roughness, and thickness were evaluated by AFM; SAMP film thickness and the structural contributions of each unit in the duplex were measured by X-ray reflection (XRR). The duplex was compared with the aliphatic and aromatic monolayer SAMPs to determine the effect of stacking on electrochemical properties; these were measured by impedance spectroscopy using aqueous electrolytes in the frequency range 20 Hz to 100 kHz, and data were analyzed using resistance-capacitance network based equivalent circuits. For the 11-hydroxyundecylphosphonate SAMP, C(SAMP) = 2.6 ± 0.2 μF/cm(2), consistent with its measured layer thickness (ca. 1.1 nm). For the anthracene-based SAMPs, C(SAMP) = 6-10 μF/cm(2), which is attributed primarily to a higher effective dielectric constant for the aromatic moieties (ε = 5-10) compared to the aliphatic one; impedance spectroscopy measured the additional capacitance of the second aromatic monolayer in the duplex (2ndSAMP) to be C(Ti/2ndSAMP) = 6.8 ± 0.7 μF/cm(2), in series with the first.

Background: The chemomechanical properties of myosin-XXI, seemingly the only myosin expressed in Leishmania parasites, are unknown. Results: Recombinantly expressed full-length myosin-XXI is an active ATPase and, in the presence of calmodulin, moves actin filaments. Conclusion: Myosin-XXI is a mechanically functional molecular motor. Significance: Calmodulin-dependent regulation of myosin-XXI might be involved in the various functions of the motor in the parasite lifecycle. The genome of the Leishmania parasite contains two classes of myosin. Myosin-XXI, seemingly the only myosin isoform expressed in the protozoan parasite, has been detected in both the promastigote and amastigote stages of the Leishmania life cycle. It has been suggested to perform a variety of functions, including roles in membrane anchorage, but also long-range directed movements of cargo. However, nothing is known about the biochemical or mechanical properties of this motor. Here we designed and expressed various myosin-XXI constructs using a baculovirus expression system. Both full-length (amino acids 1–1051) and minimal motor domain constructs (amino acids 1–800) featured actin-activated ATPase activity. Myosin-XXI was soluble when expressed either with or without calmodulin. In the presence of calcium (pCa 4.1) the full-length motor could bind a single calmodulin at its neck domain (probably amino acids 809–823). Calmodulin binding was required for motility but not for ATPase activity. Once bound, calmodulin remained stably attached independent of calcium concentration (pCa 3–7). In gliding filament assays, myosin-XXI moved actin filaments at ∼15 nm/s, insensitive to both salt (25–1000 mm KCl) and calcium concentrations (pCa 3–7). Calmodulin binding to the neck domain might be involved in regulating the motility of the myosin-XXI motor for its various cellular functions in the different stages of the Leishmania parasite life cycle.

We present the addition of an energy relay dye to fullerenes resulting in increased light harvesting and significantly improved power conversion efficiency for organic photovoltaic (OPV) devices. Although exhibiting excellent properties as electron acceptors, visible light absorption of fullerenes is limited. Strongly light absorbing donor materials are needed for efficient light harvesting in the thin active layer of OPV devices. Therefore, photocurrent generation and thus power conversion efficiency of this type of solar cell is confined by the overlap of the relatively narrow absorption band of commonly used donor molecules with the solar spectrum. Herein the concept of fullerene dye sensitization is presented, which allows increased light harvesting on the electron acceptor side of the heterojunction. The concept is exemplarily shown for an UV absorbing small molecule and a near infrared absorbing polymer, namely hexa-peri -hexabenzocoronene (HBC) and Poly[ 2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b: 3,4-b'] dithiophene-2,6-diyl]] (PCPDTBT), respectively. In both systems remarkably higher power conversion efficiency is achieved via perylene sensitization of the fullerene acceptor. Steady state photoluminescence, transient absorption and transient photocurrent decay studies reveal pathways of the additionally generated excited states at the sensitizer molecule. The findings suggest fluorescence resonance energy transfer from the photo-excited dye to the fullerene enabling decoupling of light absorption and charge transport. The presented sensitization method is proposed as a viable new concept for performance enhancement in organic photovoltaic devices.