Phosphorylation in vivo and in vitro of human histocompatibility antigens (HLA-A and HLA-B) in the carboxy-terminal intracellular domain.

HLA-A and -B antigens are phosphorylated in transformed lymphoblastoid cells and peripheral blood lymphocytes, both incubated with 32Pi. The phosphate group is attached to HLA-A and -B heavy chain (p44) as identified by immunoprecipitation with anti-beta2-microglobulin IgG, sodium dodecyl sulfate/polyacrylamide gel electrophoresis, isoelectric focusing, and susceptibility to limited proteolysis by papain and trypsin. The site(s) of phosphorylation is identified as a serine residue(s) located in the hydrophilic carboxy terminus of the p44 chain. HLA antigens are also phosphorylated in isolated membranes from transformed lymphoblastoid cells that are incubated with [gamma32P]ATP. The phosphorylation of the carboxy terminus of HLA-A and -B antigens in vivo is good evidence that this portion of the molecule is intracellular. Furthermore, this modification suggests a general way in which interactions between membrane proteins and cytoskeletal elements may be regulated.