Roles of protein and carbohydrate in glycoprotein processing and secretion. Studies using mutants expressing altered IgM mu chains.

The role of specific structural elements in glycoprotein metabolism and secretion was studied using the IgM mu chains from normal and mutant hybridoma cell lines. Five classes of altered mu chains were studied, all of which lacked various portions of the normal protein sequence, and three of which had one and the others two fewer carbohydrate units than wild type mu. One mutant secreted mu chains more rapidly than wild type cells. Two of the mutant cell lines secreted very little IgM, and both of these degraded intracellular mu chains at an abnormally high rate. Tunicamycin largely blocked IgM secretion in wild type and three mutant cell lines, but caused less inhibition in the two other mutant lines. When glycosylation in the two low secreting mutants was blocked by tunicamycin, degradation of mu chains was substantially reduced in one but unaffected in the other. All of the above properties were retained by the altered mu chains when the mutants were further hybridized with cells producing wild type mu. Overall, the various carbohydrate units and polypeptide sequences seem to play different roles in a single protein's metabolism and expression. The carbohydrate moieties may exert independent effects on protein degradation and on secretion.