Association of human gamma chain with class II transplantation antigens during intracellular transport

Cell surface expressed human and murine class II transplantation antigens are composed of two polypeptide chains called alpha and beta. During intracellular transport an invariant chain, provisionally called gamma, is associated with the class II antigen chains. Since gamma chains leave the endoplasmic reticulum only when associated with alpha and beta chains, we have studied the intracellular transport of the gamma chain and its possible cell surface expression. Modifications of the carbohydrate moieties of the gamma chain during intracellular transport were also examined. The gamma chain appears to contain two Asn-linked carbohydrate moieties and maybe also one or more Ser/Thr-linked carbohydrates. At all times during the pulse-chase experiments core glycosylated gamma chains resolved into two distinct spots on two-dimensional gel electrophoresis. The occurrence of core-glycosylated gamma chains was expected since more gamma chains than alpha and beta chains exist in the endoplasmic reticulum. Terminally glycosylated, alpha, beta, and gamma chains emerged simultaneously supporting the idea that the three types of chains are brought to the Golgi complex bound to each other. However, terminal glycosylation is temporally related to the dissociation of the gamma chain from the alpha and beta chains. Since isolated plasma membranes contained molecules indistinguishable from gamma chains, it is concluded that gamma chains are transported together with class II antigens from the endoplasmic reticulum to the Golgi complex. After dissociation, class II antigens and some, if not all, gamma chains seem to become independently integrated into the plasma membrane.