title: Transglutaminase 2 kinase activity facilitates protein kinase A-induced phosphorylation of retinoblastoma protein.
abstract: Transglutaminase 2 (TG2, tissue transglutaminase) is a multifunctional protein involved in cross-linking a variety of proteins, including retinoblastoma protein (Rb). Here we show that Rb is also a substrate for the recently identified serine/threonine kinase activity of TG2 and that TG2 phosphorylates Rb at the critically important Ser780 residue. Furthermore, phosphorylation of Rb by TG2 destabilizes the Rb.E2F1 complex. TG2 phosphorylation of Rb was abrogated by high Ca2+ concentrations, whereas TG2 transamidating activity was inhibited by ATP. TG2 was itself phosphorylated by protein kinase A (PKA). Phosphorylation of TG2 by PKA attenuated its transamidating activity and enhanced its kinase activity. Activation of PKA in mouse embryonic fibroblasts (MEF) with dibutyryl-cAMP enhanced phosphorylation of both TG2 and Rb by a process that was inhibited by the PKA inhibitor H89. Treatment with dibutyryl-cAMP enhanced Rb phosphorylation in MEFtg2+/+ cells but not in MEFtg2-/- cells. These data indicate that Rb is a substrate for TG2 kinase activity and suggest that phosphorylation of Rb, which results from activation of PKA in fibroblasts, is indirect and requires TG2 kinase activity.
authors.affiliation: Departments of Physiology and Internal Medicine, University of Manitoba, Winnipeg R3E 0W3, Canada. Electronic address: mishra@cc.umanitoba.ca.
authors.forename: Suresh
authors.initials: S
authors.lastname: Mishra
authors.affiliation: Department of Biochemistry Laboratory, Instituto Dermapatico dell'Immaculata-Institute for Hospitalization, Care and Scientific Research, Department of Experimental Medicine and Biological Sciences, University of Rome, 00133 Tor Vergata, Italy.
authors.forename: Gerry
authors.initials: G
authors.lastname: Melino
authors.affiliation: Departments of Physiology and Internal Medicine, University of Manitoba, Winnipeg R3E 0W3, Canada.
authors.forename: Liam J
authors.initials: LJ
authors.lastname: Murphy
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D017209:Apoptosis; D002118:Calcium / Q000378:metabolism*; D002453:Cell Cycle; D002460:Cell Line; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism*; D005347:Fibroblasts / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000378:metabolism*; D006801:Humans; D051379:Mice; D010766:Phosphorylation; D000091345:Protein Glutamine gamma Glutamyltransferase 2; D016160:Retinoblastoma Protein / Q000378:metabolism*; D016196:S Phase; D012694:Serine / Q000737:chemistry; D011503:Transglutaminases / Q000378:metabolism*
chemical_list: D016160:Retinoblastoma Protein; D012694:Serine; D000255:Adenosine Triphosphate; D000091345:Protein Glutamine gamma Glutamyltransferase 2; D011503:Transglutaminases; D017868:Cyclic AMP-Dependent Protein Kinases; D019204:GTP-Binding Proteins; D002118:Calcium