B lymphocyte differentiation and the control of IgM mu chain expression

The biosynthesis of IgM mu polypeptides was studied in isolated populations of normal B lymphocytes and in various IgM-producing cell lines. Membrane and secretory mu were found to be distinct polypeptide species, with separate biosynthetic intermediates from the translation stage onwards. Various B cell populations express different portions of the two biosynthetic mu pathways. Normal, resting small B lymphocytes do not secrete detectable mu and lack the ater intermediate forms of secretory mu. However, they apparently possess, and translate, secretory mu mRNA, and show earlier secretory mu intermediate protein forms. Resting B cells thus exert posttranslational control over secretory mu expression. Since the later intermediate forms of secretory mu, which are lacking in small B cells, are due to carbohydrate modifications of the mu chain, it is suggested that the carbohydrate portion may be involved in regulating the expression of the secretory mu glycoprotein. In contrast to small B cells, highly differentiated IgM-secreting cells control the expression of membrane mu by a pretranslational mechanism.