THE ALPHA BETA SHEATH AND ITS CYTOPLASMIC TYROSINES ARE REQUIRED FOR SIGNALING BY THE B-CELL ANTIGEN RECEPTOR BUT NOT FOR CAPPING OR FOR SERINE THREONINE-KINASE RECRUITMENT/

The B-cell antigen receptor is composed of membrane immunoglobulin she athed by an alpha/beta heterodimer. The complex is noncovalently assoc iated with protein kinase activity, and crosslinking of the receptor l eads to capping and transmembrane signaling. Here we show that the she ath is not necessary either for this capping or for the association of membrane immunoglobulin with the detergent-insoluble cytoskeletal fra ction that occurs following crosslinking. It is also not required for association of membrane immunoglobulin with a casein-kinase-like serin e/threonine kinase. The sheath is essential, however, for transmembran e signaling. Provision of just the cytoplasmic domain of the beta shea th polypeptide to a mutant, unsheathed IgM molecule was sufficient to restore full signaling capability as judged by the phosphorylation of a variety of cellular proteins, including the B-cell-specific transmem brane protein CD22. This signaling was destroyed by mutating one of th e tyrosines in the beta cytoplasmic domain. These results not only sug gest that receptor signaling is mediated through phosphorylation of th e tyrosines in the sheath's cytoplasmic domains but, together with pre vious work, indicate that different moths within the sheath mediate pr esentation and signaling.