Family with sequence similarity member 20C is the primary but not the only kinase for the small-integrin-binding ligand N-linked glycoproteins in bone.

Recent studies have identified family with sequence similarity member 20C (FAM20C) as a kinase that phosphorylates the Ser in Ser-X-Glu/phospho-Ser (pSer) motifs in the small-integrin-binding ligand N-linked glycoproteins (SIBLINGs). There is no in vivo evidence that validates this finding, and it is unclear whether FAM20C is the only kinase for SIBLINGs. We extracted bone noncollagenous proteins (NCPs) from Fam20C-knockout (KO) mice and analyzed the phosphorylation levels. The total NCPs were separated into osteopontin-, bone sialoprotein-, and dentin matrix protein-1-enriched fractions by anion-exchange chromatography and analyzed by SDS-PAGE, native PAGE, and Western immunoblot analysis. The NCP phosphorylation level in the KO mice was lower than that in the wild-type (WT). On the native gel, the SIBLINGs from KO mice showed a lower migration rate (Mr) than those from the WT. Calf intestine phosphatase treatment shifted SIBLINGs from the WT mice to the level adjacent to the KO, but failed to shift the latter, suggesting a phosphorylation loss of SIBLINGs in the KO mice. Mass spectrometry identified less pSers in the SIBLINGs from the KO mice [including the region of the acidic Ser- and aspartate-rich motif (ASARM) peptides]. In an intriguing finding, several pSers in the Ser-X-Glu motifs in the KO mice maintained their phosphorylation, whereas several others in non-Ser-X-Glu motifs did not. Phospho-Tyrs and phospho-Thrs in the SIBLINGs did not appear to be associated with FAM20C. Our results indicate that FAM20C is the primary, but not the only, kinase for the SIBLINGs.-Yang, X., Yan, W., Tian, Y., Ma, P., Opperman, L. A., Wang, X. Family with sequence similarity member 20C is the primary but not the only kinase for the small-integrin-binding ligand N-linked glycoproteins in bone.