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 unconventional myosin heavy chain, human myosin-IXb, is described. The primary structure of this myosin predicts a 229 kDa protein that together with its recently described rat homolog, myr 5, is the ninth class of myosins to be identified. In comparison to skeletal muscle myosin-II, the myosin-IXb 'head' has two unusual features: a novel N-terminal domain of 140 amino acids, which includes a 60 amino acid extension, and a large insertion of 126 amino acids in the putative actin-binding site. The 'neck' contains four tandemly repeated IQ motifs, suggesting that this myosin may have four associated light chains. The 'tail' contains a region similar to regions found in the chimerins, with a putative zinc and diacylglycerol binding domain, homologous to the regulatory domain of protein kinase C and a putative GTPase-activating protein (GAP) domain of the rho/rac family of ras-like G-proteins. Northern blot analysis of 16 different human tissues revealed an approximately 8 kb transcript that is most highly expressed in peripheral blood leukocytes, with somewhat lower levels of expression in thymus and spleen, suggesting that myosin-IXb is most abundant in cells of myeloid origin. Myosin-IXb was also expressed in a number of other tissues at significantly lower levels. Analysis of myosin-IXb protein expression, using a tail-domain directed antibody, was performed in HL-60 cells, a human leukocyte cell. Myosin-IXb expression increases by 4- to 5-fold upon induced differentiation of these cells into macrophage-like cells. The localization of myosin-IXb is also altered upon differentiation. In undifferentiated HL-60 cells, myosin-IXb colocalizes with F-actin in the cell periphery, while in differentiated cells its localization becomes more cytoplasmic, with the highest levels in the perinuclear region.
Department of Biology, School of Medicine, Yale University, New Haven, CT 06520, USA.
J A
JA
Wirth
K A
KA
Jensen
P L
PL
Post
W M
WM
Bement
M S
MS
Mooseker
D000595:Amino Acid Sequence; D001483:Base Sequence; D002460:Cell Line; D020690:GTPase-Activating Proteins; D006801:Humans; D007962:Leukocytes / Q000378:metabolism; D008969:Molecular Sequence Data; D018995:Myosin Heavy Chains / Q000737:chemistry*; D017434:Protein Structure, Tertiary; D011506:Proteins / Q000737:chemistry*; D020703:ras GTPase-Activating Proteins
D020690:GTPase-Activating Proteins; D011506:Proteins; D020703:ras GTPase-Activating Proteins; D018995:Myosin Heavy Chains