Recruitment of a specific amoeboid myosin I isoform to the plasma membranein chemotactic Dictyostelium cells

The Dictyostelium class I myosins, MyoA, -B, -C, and -D, participate in pla sma membrane-based cellular processes such as pseudopod extension and macro pinocytosis. Given the existence of a high affinity membrane-binding site i n the C-terminal tail domain of these motor proteins and their localized si te of action at the cortical membrane-cytoskeleton, it was of interest to d etermine whether each myosin I was directly associated with the plasma memb rane. The membrane association of a myosin I heavy chain kinase that regula tes the activity of one of the class I myosins, MyoD was also examined. Cel lular fractionation experiments revealed that the majority of the Dicyostel ium MyoA, -B, -C and -D heavy chains and the kinase are cytosolic. However, a small, but significant, fraction (appr, 7. -15%) of each myosin I and th e kinase was associated with the plasma membrane. The level of plasma membr ane-associated MyoB, but neither that of MyoC nor MyoD, increases up to 2-f old in highly motile, streaming cells. These results indicate that Dictyost elium specifically recruits myoB to the plasma membrane during directed cel l migration, consistent with its known role in pseudopod formation.