FAM20C phosphorylation of the RGDSVVYGLR motif in osteopontin inhibits interaction with the αvβ3 integrin.

Osteopontin (OPN) is a ubiquitously expressed, multifunctional, and highly phosphorylated protein. OPN contains two neighboring integrin-binding motifs, RGD and SVVYGLR, which mediate interaction with cells. Phosphorylation and proteolytic processing affect the integrin-binding activities of OPN. Here we report that the kinase, FAM20C, phosphorylates Ser in the RGDSVVYGLR motif of OPN and that Ser is phosphorylated in vivo in human and bovine milk. Ser is located right next to the RGD motif and close by the regulatory thrombin and plasmin cleavage sites in the OPN sequence. Phosphorylation of Ser could potentially affect the proteolytic processing and the integrin-binding activities of OPN. We show that phosphorylation of Ser does not affect the susceptibility of OPN for thrombin or plasmin cleavage. However, phosphorylation of Ser significantly reduces the RGD-mediated interaction with the α β integrin in MDA-MB-435 and Moαv cells. This suggests a new mechanism by which specific phosphorylation of OPN can regulate interaction with the α β integrin and thereby affect OPN-cell interaction.