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J Biol Chem, Vol. 273, Issue 43, 27809-27815, October 23, 1998

Internalization of the Lymphocytic Surface Protein CD22 Is Controlled by a Novel Membrane Proximal Cytoplasmic Motif

Claude H. T. Chan, Junsheng Wang, Ruth R. French, and Martin J. Glennie

From the Lymphoma Research Unit, Tenovus Cancer Laboratory, Southampton General Hospital, Tremona Rd., Southampton SO16 6YD, United Kingdom and the  Medical Research Council Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, United Kingdom

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 cDNA vectors were constructed and stably transfected in CD22-negative Jurkat T-lymphocytic cells. Two assays were employed to measure CD22 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 × anti-saporin]) antibody. Results showed that CD22 lacking a cytoplasmic tail was not internalized and that replacement of the cytoplasmic tail of CD19 with that of CD22 resulted in a chimeric molecule that behaved like CD22 and internalized rapidly. Step-wise deletion of the cytoplasmic tail of CD22 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.

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