THE SPECIFICITY OF ASSOCIATION OF THE IGD MOLECULE WITH THE ACCESSORYPROTEINS BAP31 BAP29 LIES IN THE IGD TRANSMEMBRANE SEQUENCE/

Mature B cells co-express on their cell surface two classes of antigen receptor, the IgM and IgD immunoglobulins. The structural and functio nal differences between the two receptor classes are poorly understood , Recently two proteins of 29 and 31 kDa (BAP29 and BAP31) have been d escribed that are preferentially associated with membrane IgD but only weakly with membrane IgM. We describe here the cloning of full-length murine and human BAP31 cDNAs encoding proteins of 245 and 246 amino a cids respectively, The two BAP31 proteins are 95% identical, The BAP31 gene is ubiquitously expressed in murine tissues and is located on th e X chromosome in both mouse and man, The murine BAP31 protein has 43% sequence identity to murine BAP29, Both proteins have a hydrophobic N -terminus and an alpha-helical C-terminus which ends with a KKXX motif implicated in vesicular transport. By a mutational analysis we have i dentified amino acids in the transmembrane sequence of the delta m cha in that are critical for binding to BAP31/BAP29. A structural model of the BAPs and their potential functions are discussed.