title: Conserved phosphorylation of serines in the Ser-X-Glu/Ser(P) sequences of the vitamin K-dependent matrix Gla protein from shark, lamb, rat, cow, and human.
abstract: The present studies demonstrate that matrix Gla protein (MGP), a 10-kDa vitamin K-dependent protein, is phosphorylated at 3 serine residues near its N-terminus. Phosphoserine was identified at residues 3, 6, and 9 of bovine, human, rat, and lamb MGP by N-terminal protein sequencing. All 3 modified serines are in tandemly repeated Ser-X-Glu sequences. Two of the serines phosphorylated in shark MGP, residues 2 and 5, also have glutamate residues in the n + 2 position in tandemly repeated Ser-X-Glu sequences, whereas the third, shark residue 3, would acquire an acidic phosphoserine in the n + 2 position upon phosphorylation of serine 5. The recognition motif found for MGP phosphorylation, Ser-X-Glu/Ser(P), has been seen previously in milk caseins, salivary proteins, and a number of regulatory peptides. A review of the literature has revealed an intriguing dichotomy in the extent of serine phosphorylation among secreted proteins that are phosphorylated at Ser-X-Glu/Ser(P) sequences. Those phosphoproteins secreted into milk or saliva are fully phosphorylated at each target serine, whereas phosphoproteins secreted into the extracellular environment of cells are partially phosphorylated at target serine residues, as we show here for MGP and others have shown for regulatory peptides and the insulin-like growth factor binding protein 1. We propose that the extent of serine phosphorylation regulates the activity of proteins secreted into the extracellular environment of cells, and that partial phosphorylation can therefore be explained by the need to ensure that the phosphoprotein be poised to gain or lose activity with regulated changes in phosphorylation status.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Biology, University of California at San Diego, La Jolla 92093-0322.
authors.forename: P A
authors.initials: PA
authors.lastname: Price
authors.forename: J S
authors.initials: JS
authors.lastname: Rice
authors.forename: M K
authors.initials: MK
authors.lastname: Williamson
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D002417:Cattle; D017124:Conserved Sequence; D006801:Humans; D008954:Models, Biological; D008969:Molecular Sequence Data; D015675:Osteocalcin / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D010766:Phosphorylation; D010768:Phosphoserine / Q000737:chemistry; D011494:Protein Kinases / Q000378:metabolism; D051381:Rats; D017386:Sequence Homology, Amino Acid; D012694:Serine / Q000737:chemistry / Q000235:genetics; D012754:Sharks; D012756:Sheep; D013045:Species Specificity; D013379:Substrate Specificity
chemical_list: D015675:Osteocalcin; D010768:Phosphoserine; D012694:Serine; D011494:Protein Kinases