The modification of glutamic acid residues to g-carboxyglutamic acid (Gla) is a post-translational modification catalyzed by the vitamin K-dependent γ-glutamylcarboxylase enzyme. Despite ubiquitous expression of the γ-carboxylation machinery in mammalian tissues, only 12 Gla-containing proteins have so far been identified in humans. Because bone tissue is the second most abundant source of Gla-proteins after the liver, we sought to identify Gla proteins secreted by bone-marrow derived mesenchymal stromal cells (MSCs), a precursor to all non-hematopoietic cells in bones. We used a proteomics approach to screen the secretome of MSCs with a combination of 2D gel electrophoresis and tandem mass spectrometry. The most abundant Gla-protein secreted by MSCs was identified as periostin, a previously unrecognized γ-carboxylated protein. In silico aminoacid sequence analysis of periostin demonstrated the presence of four consensus γ-carboxylase recognition sites embedded within fasciclin-like protein domains. The carboxylation of periostin was confirmed by immunoprecipitation using anti-Gla antibodies and could be inhibited by warfarin in MSCs. In conclusion, periostin is a novel vitamin K-dependent γ-carboxylated protein distinguished from other Gla-proteins by the presence of multiple γ-carboxylase recognition sites and MSCs are an abundant source of periostin including its γ-carboxylated variant.
Disclosure: No relevant conflicts of interest to declare.