INTERNALIZATION OF THE LYMPHOCYTIC SURFACE PROTEIN CD22 IS CONTROLLEDBY A NOVEL MEMBRANE PROXIMAL CYTOPLASMIC MOTIF

CD22 is a key receptor on B-lymphocytes that modulates signaling durin g antigenic stimulation. We have defined a novel cytoplasmic motif in human CD22 that controls its unusually rapid turnover at the plasma me mbrane. Chimeric and mutated CD22 alpha cDNA vectors were constructed and stably transfected in CD22-negative Jurkat T-lymphocytic cells. Tw o assays were employed to measure CD22 alpha internalization: first, c ytoplasmic uptake of radioiodinated anti-CD22 monoclonal antibody; and second, lethal targeting of a toxin, saporin, into cells via CD22 usi ng bispecific F(ab')(2) ([anti-CD22 x anti-saporin]) antibody, Results showed that CD22 alpha lacking a cytoplasmic tail was not internalize d and that replacement of the cytoplasmic tail of CD19 with that of CD 22 alpha resulted in a chimeric molecule that behaved like CD22 alpha and internalized rapidly. Step-wise deletion of the cytoplasmic tail o f CD22 alpha located the internalization motif to a polar region of 11 residues (QRRWKRTQSQQ) proximal to the plasma membrane, a part of the molecule predicted to form a coil or turn structure; Interestingly, a dditional CD22 mutants showed that the two glutamine residues sandwich ing the serine are critical to internalization but that the serine its elf is not.