CD22 Forms a Quaternary Complex with SHIP, Grb2, and Shc

Abstract

CD22 is a cell surface molecule that regulates signal transduction in B lymphocytes. Tyrosine-phosphorylated CD22 recruits numerous cytoplasmic effector molecules including SHP-1, a potent phosphotyrosine phosphatase that down-regulates B cell antigen receptor (BCR)- and CD19-generated signals. Paradoxically, B cells from CD22-deficient mice generate augmented intracellular calcium responses following BCR ligation, yet proliferation is decreased. To understand further the mechanisms through which CD22 regulates BCR-dependent calcium flux and proliferation, interactions between CD22 and effector molecules involved in these processes were assessed. The adapter proteins Grb2 and Shc were found to interact with distinct and specific regions of the CD22 cytoplasmic domain. Src homology-2 domain-containing inositol polyphosphate-5'-phosphatase (SHIP) also bound phosphorylated CD22, but binding required an intact CD22 cytoplasmic domain. All three molecules were bound to CD22 when isolated from BCR-stimulated splenic B cells, indicating the formation of a CD22.Grb2.Shc.SHIP quaternary complex. Therefore, SHIP associating with CD22 may be important for SHIP recruitment to the cell surface where it negatively regulates calcium influx. Although augmented calcium responses in CD22-deficient mice should facilitate enhanced c-Jun N-terminal kinase (JNK) activation, BCR ligation did not induce JNK activation in CD22-deficient B cells. These data demonstrate that CD22 functions as a molecular "scaffold" that specifically coordinates the docking of multiple effector molecules, in addition to SHP-1, in a context necessary for BCR-dependent SHIP activity and JNK stimulation.