BCMAP - AN INTEGRAL MEMBRANE-PROTEIN IN THE GOLGI-APPARATUS OF HUMAN MATURE B-LYMPHOCYTES

BCMA is a human gene expressed preferentially in mature a lymphocytes as a 1.2 kb mRNA, which encodes a 184 amino acid peptide (BCMAp). The study of BCMA mRNA expression, using human malignant a cell lines char acteristic of different stages of a lymphocyte differentiation, demons trated that the BCMA mRNA is absent in the pro-B lymphocyte stage. It is expressed faintly at the pre-B cell stage and its expression increa ses with a lymphocyte maturation. Polyclonal antibodies were used to s how, by cellular fractionation and immunoprecipitation, that BCMAp is a non-glycosylated integral membrane protein. Furthermore, BCMAp inser ts, in vitro, into canine microsomes, as a type I integral membrane pr otein. Cell surface labeling showed that BCMAp is not expressed in the plasma membrane of mature a lymphocytes. Immunofluorescence studies r evealed that BCMAp lies in a cap-like structure near the nucleus, that was identified as the Golgi apparatus by co-localization of BCMAp wit h CTR433, a marker of the medial cisternae of the Golgi apparatus, Con focal scanning laser microscopy of U266 plasma cells labeled with mark ers of various Golgi apparatus subcompartments strongly suggests that BCMAp is located in the cia part of the Golgi apparatus, Thus, BCMAp i s the first Golgi resident protein with a tissue specificity and whose expression is linked to the stage of differentiation of a lymphocytes . The location of BCMAp in the Golgi apparatus and its high expression in plasmocytes (secreting large amounts of Ig) suggest that BCMAp is implicated in the intracellular traffic of Ig.