Secreted Kinase Phosphorylates Extracellular Proteins That Regulate Biomineralization

The Real McCoy Some secreted proteins are phosphorylated, the most prominent example being the milk protein casein, but the enzymes that catalyze such phosphorylation have not been identified. (The proteins known as “casein kinases” are in fact cytosolic proteins and do not mediate physiological phosphorylation of casein.) Tagliabracci et al. (p. 1150 , published online 10 May) searched for a human protein with the characteristics expected of a secretory protein kinase and identified Fam20C. Mutations in the gene encoding Fam20C cause defects in bone formation. Furthermore, the consensus sequence for Fam20C phosphorylation was found in several secreted proteins that function in biomineralization. Thus, Fam20C appears to be the “real” casein kinase and to function in bone physiology.