Phosphorylation of osteopontin by Golgi apparatus casein kinase.
Osteopontin (OPN) is a ubiquitous multiphosphorylated secretory glycoprotein. Twenty-seven phosphorylated serines have been identified in bovine milk OPN (E. S. Sorensen et al. (1995) Protein Sci. 4, 2040-2049). Nineteen of these phosphoacceptor sites are fully conserved in rat OPN, all displaying the consensus for the Golgi apparatus casein kinase, G-CK (S-x-E/Sp). Here we show that rat OPN is indeed phosphorylated more readily than casein itself by G-CK from either rat mammary gland or liver. OPN is also phosphorylated by casein kinases-1 and -2 (CK1, CK2), though less readily than casein. If OPN kinase activities are normalized in terms of casein phosphorylation, OPN phosphorylation rate by G-CK is 78-fold and 19-fold higher than those measured with CK2 and CK1, respectively. These data, in conjunction with the specific location of G-CK to the Golgi apparatus, where CK2 and CK1 are hardly detectable, support the view that G-CK is the main if not the only physiological agent committed to the phosphorylation of OPN.
Lasa
M
M
Dipartimento di Chimica Biologica, Universita di Padova, Italy.
Chang
P L
PL
Prince
C W
CW
Pinna
L A
LA
D000595:Amino Acid Sequence; D000818:Animals; D047388:Casein Kinases; D002364:Caseins / Q000378:metabolism; D017124:Conserved Sequence; D004591:Electrophoresis, Polyacrylamide Gel; D005260:Female; D006023:Glycoproteins / Q000737:chemistry / Q000378:metabolism; D006056:Golgi Apparatus / Q000201:enzymology*; D007700:Kinetics; D008099:Liver / Q000201:enzymology; D008321:Mammary Glands, Animal / Q000201:enzymology; D008969:Molecular Sequence Data; D053495:Osteopontin; D010761:Phosphorus Radioisotopes / Q000378:metabolism; D010766:Phosphorylation; D011494:Protein Kinases / Q000378:metabolism*; D051381:Rats; D011994:Recombinant Proteins / Q000302:isolation & purification / Q000378:metabolism; D012795:Sialoglycoproteins / Q000378:metabolism*; D013379:Substrate Specificity
D002364:Caseins; D006023:Glycoproteins; D010761:Phosphorus Radioisotopes; D011994:Recombinant Proteins; D012795:Sialoglycoproteins; C506897:Spp1 protein, rat; D053495:Osteopontin; D011494:Protein Kinases; D047388:Casein Kinases