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.
Wirth
J A
JA
Department of Biology, School of Medicine, Yale University, New Haven, CT 06520, USA.
Jensen
K A
KA
Post
P L
PL
Bement
W M
WM
Mooseker
M S
MS
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