Surface transport and internalization of the membrane IgM H chain in the absence of the Mb-1 and B29 proteins.

The micron IgH chain is one component of the membrane IgM complex, the endocytic and signal transducing receptor for Ag on the surface of B lymphocytes. It is transported to the cell surface in association with three other B cell-specific proteins, an L chain and the products of the mb-1 and B29 genes. The roles played by these various proteins in mediating the functions of the complex are unclear. To analyze micron function in the absence of other lymphoid-specific proteins, we first attempted to express micron on the surface of nonlymphoid cells. Deletion of the CH1 domain was sufficient to allow surface expression of this protein in transfected COS cells as well as in mouse L cells. To determine whether this extracellularly truncated micron was capable of endocytosis, we used an assay to detect the internalization of anti-mu antibody bound to the surface of transfected cells expressing the protein. Under both cross-linking and non cross-linking conditions, the CH1-deleted micron protein was internalized in endocytic vesicles. We conclude from these observations that a) the CH1 domain of micron contains a retention signal, the elimination of which allows surface transport of this protein, and b) micron by itself is capable of at least one of the functions of the membrane IgM complex.