RESCUE OF IMMUNOGLOBULINS FROM INSOLUBILITY IS FACILITATED BY PDI IN THE BACULOVIRUS EXPRESSION SYSTEM

A substantial fraction of immunoglobulin heavy and light chain polypep tides were insoluble when expressed in the baculovirus-insect cell exp ression system. In the presence of coexpressed heterologous protein di sulfide isomerase (PDI), however, the solubility of the immunoglobulin s was enhanced and IgG was secreted at higher levels from baculovirus- infected Trichoplusia ni insect cells. Pulse-chase experiments indicat ed that some immunoglobulin polypeptides were initially insoluble in t he presence of PDI but subsequently were rescued in a soluble form com petent for IgG assembly and secretion. Recovery of the insoluble immun oglobulins was not observed in the absence of coexpressed PDI. Even af ter treatment of insect cells with tunicamycin to inhibit N-glycosylat ion of immunoglobulin heavy chains, coexpressed PDI was able to salvag e insoluble immunoglobulins and secrete these modified glycoforms. The capacity for PDI to rescue immunoglobulins was also demonstrated in v itro where immunoglobulin heavy chains and light chain dimers were sal vaged from aggregates of denatured IgG. PDI-mediated rescue of protein s, perhaps assisted by chaperones and other foldases, may be important in vivo where insolubility is a common occurrence for newly synthesiz ed polypeptides. (C) 1996 Academic Press, Inc.