HUMAN MYOSIN-IXB, AN UNCONVENTIONAL MYOSIN WITH A CHIMERIN-LIKE RHO RAC GTPASE-ACTIVATING PROTEIN DOMAIN IN ITS TAIL/

The full-length primary structure and expression profile of a novel un conventional myosin heavy chain, human myosin-Mb, is described. The pr imary structure of this myosin predicts a 229 kDa protein that togethe r with its recently described rat homolog, myr 5, is the ninth class o f myosins to be identified. In comparison to skeletal muscle myosin-II , the myosin-IXb 'head' has two unusual features: a novel N-terminal d omain of 140 amino acids, which includes a 60 amino acid extension, an d a large insertion of 126 amino acids in the putative actin-binding s ite. The 'neck' contains four tandemly repeated IQ moths, suggesting t hat this myosin may have four associated light chains. The 'tail' cont ains a region similar to regions found in the chimerins, with a putati ve zinc and diacylglycerol binding domain, homologous to the regulator y domain of protein kinase C and a putative GTPase-activating protein (GAP) domain of the rho/rac family of ras-like G-proteins. Northern bl ot analysis of 16 different human tissues revealed an similar to 8 kb transcript that is most highly expressed in peripheral blood leukocyte s, with somewhat lower levels of expression in thymus and spleen, sugg esting that myosin-IXb is most abundant in cells of myeloid origin. My osin-IXb was also expressed in a number of other tissues at significan tly lower levels. Analysis of myosin-IXb protein expression, using a t ail-domain directed antibody, was performed in HL-60 cells, a human le ukocyte cell. Myosin-IXb expression increases by 4- to 5-fold upon ind uced differentiation of these cells into macrophage-like cells. The lo calization of myosin-Mb is also altered upon differentiation. In undif ferentiated HL-60 cells, myosin-Mb colocalizes with F-actin in the cel l periphery, while in differentiated cells its localization becomes mo re cytoplasmic, with the highest levels in the perinuclear region.