Efficient Dissociation of the 1>88 Chaperone from Major Histocompatibility Complex Class I Molecules

Previously, we showed that an 88-kD protein (p88) associates rapidly and quantitatively with newly synthesized murine major histocompatibility complex class I molecules within the endoplasmic reticulum (ER). This interaction is transient and dissociation of p88 appears to be rate limiting for transport of class I molecules from the ER to the Golgi apparatus. In this report, we examine the relationship between p88 interaction and assembly of the ternary complex of class I heavy chain B2-microglobulin (Bzm), and peptide ligand. In both murine and human /~2m-deficient cells, in which little or no transport of class I heavy chains is observed, p88 remained associated with intracellular heavy chains throughout their lifetime. In routine RMA-S cells, which are apparently defective in accumulating peptide ligands for class I within the ER, prolonged association of p88 with "empty" heavy chain-B2m heterodimers was also observed. However, p88 dissociated slowly in parallel with the slow rate of ER to Golgi transport of empty class I molecules in these cells. The close correlation between p88 association and impaired class I transport suggests that p88 functions to retain incompletely assembled class I molecules in the ER. We propose that conformational changes in class I heavy chains induced by the binding of both Bzm and peptide are required for efficient p88 dissociation and subsequent class I transport.