Myosin-XXI, a Motor with many Missions

Abstract

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.