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

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Abstract
Cell surface expressed human and murine class II transplantation antigens are composed of 2 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, the intracellular transport of the .gamma. chain and its possible cell surface expression were studied. Modifications of the carbohydrate moieties of the .gamma. chain during intracellular transport were also examined. The .gamma. chain appears to contain 2 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 2 distinct spots on 2-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 3 types of chains are brought to the Golgi complex bound to each other. 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.