Diagrammatic representation of critical amino acid residues in the cytoplasmic tail of CD150 and alignment with mouse and monkey CD150 sequences.

The primary SAP binding motif is boxed. (1) Leu278 partially stabilizes the nontyrosine phosphorylation-dependent SAP/CD150 interaction. (2) Thr279 is critical in the SAP motif for phospho- and non–phosphotyrosine-dependent binding. (3) Tyr281 is phosphorylated byfyn and lck and binds SAP and SHP-2. It is essential for SHP-2 binding. (4) Val284 interacts with the hydrophobic SH2 domain pocket of SAP.24 (5) Tyr307 is phosphorylated by fyn and lck but does not interact with SAP. (6) Thr325 stabilizes phosphotyrosine-dependent binding of SAP to the third motif in CD150. (7) Tyr327 is phosphorylated by fyn,and weakly binds SAP. (8) Val330 interacts with the hydrophobic SH2 domain pocket of SAP.24