IDENTIFICATION, PARTIAL SEQUENCE AND GENETIC-ANALYSIS OF MLPA, A NOVEL GENE ENCODING A MYOSIN-RELATED PROTEIN IN PHYSARUM-POLYCEPHALUM

Studies of motility in Physarum polycephalum have concentrated on the well-defined actomyosin system in plasmodia. It is clear from recent g enetic studies in lower eukaryotes that myosin is involved in a number of physiological processes in addition to the contractile functions p reviously ascribed to the classical type II myosins. Moreover, the myo sin protein family has proved to be more complex than anticipated, wit h an increasing number of reported specialized isoforms. Although a my osin type II activity has been identified in both amoebae and plasmodi a of P. polycephalum, and it has been inferred that these proteins und ergo a phase-specific isoform switch during development, this phenomen on has not been analysed genetically. In an effort to understand the p utative developmental expression of actomyosin-associated proteins, we isolated a 180-kDa protein from amoebae which is highly enriched, alo ng with actin and myosin, in actomyosin preparations in the presence o f mM concentrations of Mg++ ions and 10 mM of ATP. Using polyclonal an tisera raised against p180 we have cloned and sequenced a partial cDNA encoding a protein whose predicted amino-acid sequence indicates some similarity with the Dictyostelium discoideum myosin heavy-chain tail domain. Southern-blot and RFLP analyses indicate that the gene involve d, designated mlpA (myosin-like protein), occurs in a single copy in t he genome, is a novel Physarum gene and is expressed during amoebal an d plasmodial growth and in the dormant forms of both these cell types.