Structural analysis of the myeloma-associated membrane antigen KMA.

kappa-Myeloma antigen (KMA) was immunoprecipitated from lactoperoxidase-radioiodinated HMy2 lymphoblastoid cells by using monoclonal antibody K-1-21 and was analyzed by SDS-PAGE. Under reducing conditions, two major subunits of Mr approximately 26,000 and Mr approximately 42,000, and minor components of Mr approximately 28,000, 31,000, and 36,000 were observed. The Mr approximately 26,000 subunit was identical to kappa-light chains from HMy2 surface IgG in apparent m.w., isoelectric point, and staphylococcal V-8 protease peptide map, but was not precipitated in association with Ig heavy chain. The Mr approximately 42,000 component was homologous to rabbit skeletal muscle actin by peptide mapping with staphylococcal V-8 protease. The cell surface origin of the immunoprecipitated antigen was confirmed by demonstrating lactoperoxidase dependence of iodination and complete removal from the cell surface after pronase treatment of viable cells. Thus, cell surface expression of KMA is the result of membrane association of non-heavy chain-linked kappa-light chains, possibly in noncovalent association with actin.