title: Alterations in C3 activation and binding caused by phosphorylation by a casein kinase released from activated human platelets.
abstract: A casein kinase released from activated human platelets phosphorylates a number of plasma proteins extracellularly, and that activation of platelets in systemic lupus erythematosus patients parallels an increase in the phosphate content of plasma proteins, including C3. The present study was undertaken to characterize this platelet protein kinase and to further elucidate the effect(s) on C3 function of phosphorylation by platelet casein kinase. The phosphate content of human plasma C3 was increased from 0.15 to 0.60 mol phosphate/mol of C3 after platelet activation in whole blood or platelet-rich plasma. The platelet casein kinase was distinct from other casein kinases in terms of its dependence on cations, inhibition by specific protein kinase inhibitors, and immunological reactivity. C3 that had been phosphorylated with platelet casein kinase was tested for its susceptibility to cleavage by trypsin or the classical and alternative pathway convertases and its binding to EAC and IgG. Phosphorylation did not affect the cleavage of C3 into C3a and C3b, but the binding of fragments from phosphorylated C3 to EAC14oxy2 cells and to IgG in purified systems and in serum was increased by 1.6-4.5 times over that of unphosphorylated C3. A covariation was seen between the enhanced binding of C3 fragments to IgG after phosphorylation and an increased ratio of glycerol/glycine binding, from 2.0 for unphosphorylated C3 to 4.9 for phosphorylated C3. The present study suggests that an overall effect of phosphorylation of C3 by platelet casein kinase is to enhance the opsonization of immune complexes.
authors.affiliation: Department of Clinical Immunology and Transfusion Medicine, University Hospital, Uppsala, Sweden. kristina.nilsson_ekdahl@klinimm.uu.se
authors.forename: K N
authors.initials: KN
authors.lastname: Ekdahl
authors.forename: B
authors.initials: B
authors.lastname: Nilsson
mesh_terms: D000818:Animals; D001666:Binding Sites, Antibody; D001769:Blood / Q000276:immunology; D001781:Blood Component Removal; D001792:Blood Platelets / Q000201:enzymology* / Q000276:immunology; D047388:Casein Kinases; D003167:Complement Activation / Q000276:immunology*; D003176:Complement C3 / Q000276:immunology / Q000378:metabolism*; D050577:Complement C3-C5 Convertases / Q000378:metabolism; D011415:Complement Factor B / Q000378:metabolism; D003170:Complement Pathway, Alternative / Q000276:immunology; D003171:Complement Pathway, Classical / Q000276:immunology; D005990:Glycerol / Q000378:metabolism; D005998:Glycine / Q000378:metabolism; D006801:Humans; D006868:Hydrolysis; D007074:Immunoglobulin G / Q000097:blood / Q000378:metabolism; D007527:Isoenzymes / Q000097:blood / Q000737:chemistry; D010766:Phosphorylation; D015539:Platelet Activation / Q000276:immunology*; D011485:Protein Binding / Q000276:immunology; D011494:Protein Kinases / Q000097:blood* / Q000737:chemistry; D051381:Rats; D014357:Trypsin / Q000378:metabolism
chemical_list: D003176:Complement C3; D007074:Immunoglobulin G; D007527:Isoenzymes; D011494:Protein Kinases; D047388:Casein Kinases; D050577:Complement C3-C5 Convertases; D014357:Trypsin; D011415:Complement Factor B; D005990:Glycerol; D005998:Glycine