Internalization of the lymphocytic surface protein CD22 is controlled by a novel membrane proximal cytoplasmic motif

CD22 is a key receptor on B-lymphocytes that modulates signaling during antigenic stimulation. We have defined a novel cytoplasmic motif in human CD22 that controls its unusually rapid turnover at the plasma membrane. Chimeric and mutated CD22 alpha cDNA vectors were constructed and stably transfected in CD22-negative Jurkat T-lymphocytic cells. Two assays were employed to measure CD22 alpha internalization: first, cytoplasmic uptake of radioiodinated anti-CD22 monoclonal antibody; and second, lethal targeting of a toxin, saporin, into cells via CD22 using bispecific F(ab')(2) ([anti-CD22 x anti-saporin]) antibody, Results showed that CD22 alpha lacking a cytoplasmic tail was not internalized and that replacement of the cytoplasmic tail of CD19 with that of CD22 alpha resulted in a chimeric molecule that behaved like CD22 alpha and internalized rapidly. Step-wise deletion of the cytoplasmic tail of 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, additional CD22 mutants showed that the two glutamine residues sandwiching the serine are critical to internalization but that the serine itself is not.