ZIPPER PROTEIN, A B-G PROTEIN WITH THE ABILITY TO REGULATE ACTIN MYOSIN-1 INTERACTIONS IN THE INTESTINAL BRUSH-BORDER/

We recently identified a 28-kDa protein in the intestinal brush border that resembled tropomyosin in terms of size, homology, and alpha heli cal content, This protein contained 27 heptad repeats, nearly all of w hich began with leucine, leading to its name zipper protein, Subsequen t analysis, however, indicated that both a 49-kDa and a 28-kDa immunor eactive protein existed in intestinal brush-border extracts, Using 5'- rapid amplification of cDNA ends analysis, we extended the N-terminal sequence of zipper protein to the apparent translation start site, Thi s additional sequence contained a putative transmembrane domain and tw o potential tryptic cleavage sites C-terminal to the transmembrane dom ain which would release a 28-kDa cytoplasmic protein if utilized, The additional sequence was highly homologous to members of the B-G protei n family, a family with no known function. Immunoelectron microscopy s howed that zipper protein was confined to the membrane of the microvil lus where it was in close association with brush-border myosin 1 (BBM1 ), Recombinant zipper protein (28-kDa cytoplasmic portion) blocked the binding of actin to BBM1 and inhibited actin-stimulated BBM1 ATPase a ctivity, In contrast, zipper protein had no effect on endogenous or K/ EDTA-stimulated BBM1 ATPase activity, Furthermore, zipper protein disp laced tropomyosin from binding to actin, suggesting that these homolog ous proteins bind to the same sites on the actin molecule, We conclude that zipper protein is a transmembrane protein of the B-G family loca lized to the intestinal epithelial cell microvillus, The extended cyto plasmic tail either in the intact molecule or after tryptic cleavage m ay participate in regulating the binding and, thus, activation of BBM1 by actin in a manner similar to tropomyosin.