Modulation of Annexin II Tetramer by Tyrosine Phosphorylation

Annexin II tetramer (Allt) is a Ca²⁺-dependent phospholipid-binding phosphoprotein. In cells either expressing transforming protein tyrosine kinases or treated with growth factors such as PDGF, Allt has been shown to contain increased levels of phosphotyrosine. Therefore, we have examined the effects of the in vitro phosphorylation of Allt by pp^60c-src on several activities of the protein. Allt was phosphorylated by pp^60c-src to 0.91 ± 0.07 mol of phosphate/mol of Allt (mean ± SD). The protein tyrosine phosphorylation of Allt completely inhibited the ability of the protein to bind to and bundle F-actin. In contrast, the phosphoprotein and native protein bound to purified adrenal medulla chromaffin granules with similar affinity; however, the chromaffin granule bridging activity of the phosphoprotein was abolished. The inhibition of the chromaffin granule bridging activity of the phosphoprotein could be partially reversed by the addition of millimolar Ca²⁺. Furthermore, the phosphorylation of Allt by pp60csrc inhibited the in vitro ability of this annexin to form a complex consisting of plasma membrane, chromaffin granules, and Allt. In addition to binding to biological membranes, some annexin proteins have been shown to possess carbohydrate-binding activity. Although native Allt bound to a heparin affinity column, tyrosine phosphorylation of Allt blocked the ability of the protein to bind to the heparin affinity column. These results suggest that the tyrosine phosphorylation of Allt is a negative modulator of Allt and that the dephosphorylation of Allt might be necessary for activation of the protein.