SURFACE TRANSPORT AND INTERNALIZATION OF THE MEMBRANE IGM H-CHAIN IN THE ABSENCE OF THE MB-1 AND B29 PROTEINS

The mum Ig H chain is one component of the membrane IgM complex, the e ndocytic and signal transducing receptor for Ag on the surface of B ly mphocytes. It is transported to the cell surface in association with t hree other B cell-specific proteins, an L chain and the products of th e mb-1 and B29 genes. The roles played by these various proteins in me diating the functions of the complex are unclear. To analyze mum funct ion in the absence of other lymphoid-specific proteins, we first attem pted to express mum on the surface of nonlymphoid cells. Deletion of t he CH1 domain was sufficient to allow surface expression of this prote in in transfected COS cells as well as in mouse L cells. To determine whether this extracellularly truncated mum was capable of endocytosis, we used an assay to detect the internalization of anti-mu antibody bo und to the surface of transfected cells expressing the protein. Under both cross-linking and non cross-linking conditions, the CH1-deleted m um protein was internalized in endocytic vesicles. We conclude from th ese observations that a) the CH1 domain of mum contains a retention si gnal, the elimination of which allows surface transport of this protei n, and b) mum by itself is capable of at least one of the functions of the membrane IgM complex.