VAB-9 is a tetraspan integral membrane protein that localizes to the adherens junctions of all epithelia in C. elegans and is required for F-actin organization and epidermal (hypodermal) morphology. VAB-9 localization to the cell membrane requires HMR-1/cadherin and maintained junctional distribution of VAB-9 requires HMP-1/-catenin. VAB-9 is similar to vertebrate BCMP1 (Brain cell membrane protein 1) and defines a novel subgroup of the PMP22/EMP/Claudin family of proteins. In epithelia, members of this class of proteins localize at cell-cell junctions. Claudins are the major protein component of vertebrate tight junctions. Together these findings suggest that VAB-9 is either a direct downstream effector of HMR-1 in the adherens junction pathway, or a novel tight junction-like protein. To distinguish between these possibilities, we are studying the function of VAB-9/BCMP1 in C. elegans and vertebrate epithelial cells. To determine whether VAB-9/BCMP1 function is evolutionarily conserved, we expressed BCMP1-GFP in hypodermal cells and found that BCMP1-GFP localizes at cell junctions and rescues all
vab-9 phenotypes. Expression of VAB-9/BCMP1 in L fibroblasts, MDCK cells, cultured kidney podocytes, and developing mouse kidneys was determined. BCMP1 is most strongly expressed during embryonic kidney development and localizes at podocyte and distal tubule cell junctions. During formation of cell-cell contacts and cellular junctions in cultured cells, BCMP1 localization is dynamic. Prior to the formation of cell-cell contacts, BCMP1 localization is perinuclear. Following cadherin localization at new sites of cell-cell contact, BCMP1 translocates to regions of cell-cell contact and ultimately concentrates at cell junctions, particularly at tight junctions. In L fibroblasts, which do not express cadherin, BCMP1 does not localize at cell contacts, indicating that cadherin is essential for BCMP1 membrane localization. Upon differentiation of cultured podocytes and kidneys, BCMP1 expression is diminished, suggesting that BCMP1 has a role during junctional maturation. Finally, we determined the roles of specific regions and residues of BCMP1 in localization and function in worms. BCMP1 localization to cell membranes requires the C-terminal cytoplasmic domain and junctional concentration requires C-terminal tyrosines. The amino terminal cytoplasmic domain is not required for localization or rescue of
vab-9 morphological defects, however,
vab-9 mutants expressing the N-terminal deletion derivative of BCMP1 are thin and Lon, suggesting this region negatively regulates elongation. Together our results suggest that VAB-9/BCMP1 may have similar functions in vertebrate and invertebrate epithelia.