Figure 4.

Production of truncated μ chains by Ter3 and Ter3/Ter462* mutants. (A) The indicated cell lines were grown in [³⁵S]methionine-containing medium for 30 min at 2 × 10⁷ cells/ml. The μ chain from 2 × 10⁷ cell equivalents was precipitated with anti-IgM and protein G-agarose. The immunoprecipitated material was dissolved in SDS and reduced with mercaptoethanol. The full amount was analyzed by SDS-PAGE for all cell lines except the wild type, for which only 2 × 10⁶ cell equivalents were analyzed. Immunoprecipitation was sufficiently complete, in that <25% of the total μ-specific activity could be recovered in secondary precipitations. The results were then visualized by autoradiography. (B) Predicted sizes of μ chains which result from initiation at the indicated internal AUG codons. As noted in the text, the truncated μ chains that result from initiation at Met100 or Met139 are expected to be unglycosylated, and therefore only the amino acid component has been calculated. The size listed for μ WT also represents only the polypeptide chain (62 kDa) and does not include its ∼26-kDa oligosaccharide component.