title: Bcr is a substrate for Transglutaminase 2 cross-linking activity.
abstract: BACKGROUND
  Breakpoint cluster region (Bcr) is a multi-domain protein that contains a C-terminal GTPase activating protein (GAP) domain for Rac. Transglutaminase 2 (TG2) regulates Bcr by direct binding to its GAP domain. Since TG2 has transglutaminase activity that has been implicated in the response to extreme stress, we investigated if Bcr can also act as a substrate for TG2.
  
  
  RESULTS
  We here report that activation of TG2 by calcium caused the formation of covalently cross-linked Bcr. Abr, a protein related to Bcr but lacking its N-terminal oligomerization domain, was not cross-linked by TG2 even though it forms a complex with it. A Bcr mutant missing the first 62 amino acid residues remained monomeric in the presence of activated TG2, showing that this specific domain is necessary for the cross-linking reaction. Calcium influx induced by a calcium ionophore in primary human endothelial cells caused cross-linking of endogenous Bcr, which was inhibited by the TG2 inhibitor cystamine. Treatment of cells with cobalt chloride, a hypoxia-mimetic that causes cellular stress, also generated high molecular weight Bcr complexes. Cross-linked Bcr protein appeared in the TritonX-100-insoluble cell fraction and further accumulated in cells treated with a proteasome inhibitor.
  
  
  CONCLUSIONS
  Bcr thus represents both an interacting partner under non-stressed conditions and a target of transglutaminase activity for TG2 during extreme stress.
authors.affiliation: Section of Molecular Carcinogenesis, Division of Hematology/Oncology, Ms#54, Childrens Hospital Los Angeles, 4650 Sunset Boulevard, Los Angeles CA 90027, USA.
authors.forename: Sun-Ju
authors.initials: SJ
authors.lastname: Yi
authors.forename: John
authors.initials: J
authors.lastname: Groffen
authors.forename: Nora
authors.initials: N
authors.lastname: Heisterkamp
mesh_terms: D002118:Calcium / Q000378:metabolism; D003538:Cystamine; D042783:Endothelial Cells / Q000201:enzymology; D019204:GTP-Binding Proteins / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D020690:GTPase-Activating Proteins / Q000378:metabolism; D006801:Humans; D009154:Mutation; D011485:Protein Binding; D000091345:Protein Glutamine gamma Glutamyltransferase 2; D051562:Proto-Oncogene Proteins c-bcr / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D013312:Stress, Physiological; D013379:Substrate Specificity; D011503:Transglutaminases / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*
chemical_list: C082659:ABR protein, human; D020690:GTPase-Activating Proteins; D000091345:Protein Glutamine gamma Glutamyltransferase 2; D011503:Transglutaminases; C496613:BCR protein, human; D051562:Proto-Oncogene Proteins c-bcr; D019204:GTP-Binding Proteins; D003538:Cystamine; D002118:Calcium