CD22 ASSOCIATES WITH THE HUMAN SURFACE IGM-B-CELL ANTIGEN RECEPTOR COMPLEX

The B-cell surface molecule CD22, when cross-linked, modulates signali ng through the surface IgM (sIgM)-cell receptor (BCR) complex. Here we analyzed the basis of this interaction between CD22 and the human sIg M complex. After lysis of B cells or B-cell lines in digitonin, CD22 c oimmunoprecipitated a kinase activity that in vitro-phosphorylated two polypeptides of 150 and 130 kDa on tyrosine residues. By immunoblot a nalysis with a rabbit antiserum specific for a synthetic peptide of CD 22, we found these proteins to be CD22 itself. Furthermore, the phosph orylated 150-kDa CD22 was found in the sIgM-BCR complex maintained by digitonin, along with Igalpha/mb-1, Igbeta/B29, and a 75-kDa polypepti de precipitated by an antiserum specific to protein-tyrosine kinase PT K72. CD22 is likely to be an important signaling partner in the sIgM-B CR complex since it is very rapidly and strikingly phosphorylated afte r sIgM is cross-linked and since it contains the antigen recognition h omology I (ARHI) motif, present in other antigen receptor molecules.