AN ACIDIC MODIFICATION OF THE CYTOPLASMIC DOMAIN CONTRIBUTES TO THE CHARGE HETEROGENEITY OF THE MHC CLASS-I ANTIGENS

Polypeptide phosphorylation and sialylation of the glycan moieties con tribute to the charge heterogeneity of the class I major histocompatib ility complex glycoproteins, The present study demonstrates that a uni que acidic modification unrelated to phosphorylation or glycosylation also affects the charge heterogeneity of the H2-K-k heavy chain of BW5 147 lymphoma cells. In vitro cultivation of BW5147 cells results in ch anges in charge heterogeneity of the H2-K-k heavy chains due to the un ique acidic modification. Sequential papain digestion of the 45 000 Mr H2-K-k glycoprotein yields a 42 500 M-r glycopolypeptide initially, f ollowed by production of a 39 000 M-r glycopolypeptide. Results from e xperiments designed to localize and characterize the novel acidic modi fication suggest that the modification resides in the segment of the H 2-K-k polypeptide located between the two papain cleavage sites. This portion of the polypeptide consists of the transmembrane region and pa rt of the cytoplasmic domain of the H2-K-k heavy chain, At steady stat e, 25% of the total cell surface H2-K-k possesses this modification. I n addition, the modification is mutually exclusive with the phosphoryl ation of the H2-K-k heavy chain at Ser-333. The possible biological si gnificance of the novel modification of class I antigens is discussed.