µ-Chains from a Nonsecretor B Cell Line Differ from Secreted µ-Chains at the C-Terminal End

Peptide analysis and carboxypeptidase-A digestion have been applied to compare the C-terminal primary structure of µ-chains from nonsecreting and secreting human B cell lines. The expected release of C-terminal tyrosine was seen with internally labeled secreted µ-chains; however, only a small amount of tyrosine was released from internally labeled nonsecreted µ-chains. The greater quantity released of other hydrophobic amino acids Phe, Val, and Leu, together with differences in the kinetics of release suggested that tyrosine was not the C-terminal residue. Since these other amino acids are not found near the C-terminal sequence of secreted µ-chains (-Ser-Asp-Thr-Ala-Gly-Thr-Cys-Tyr-COOH), these results indicate a structural difference between these two forms of µ-chains. Further evidence of a structural difference between these two forms of µ-chains was obtained by the analysis of cyanogen bromide peptides. The C-terminal octapeptide from myeloma and secreted internally labeled µ-chains could be isolated by Sephadex G-50 chromatography and high voltage electrophoresis. In contrast, no such peptide could be obtained from nonsecreted internally labeled µ-chains. These results also strongly indicate that nonsecreted µ-chains (i.e., membrane) possess a different C-terminal structure from those that are secreted. These data have led to the proposal of a hydrophobic C-terminal trailer sequence being responsible for the affinity of nonsecreted µ-chain for the plasma membrane.