Oligomerization of human Gadd45a protein

Gadd45a is an 18-kDa acidic protein that is induced by genotoxic and certai n other cellular stresses. The exact function of this protein is not known. However, there is evidence for its involvement in growth control, maintena nce of genomic stability, DNA repair, cell cycle control, and apoptosis. Co nsistently, Gadd45a has previously been shown to interact in vitro and/or i n vivo with a number of proteins playing central roles in these cellular pr ocesses: proliferating cell nuclear antigen, p21(Cip1/Waf1), Cdc2-CyclinB c omplex, MTK1, and histories. Adding to this complexity, we have found that Gadd45a self-associates in solution, both in vitro and when expressed in th e cell. Moreover, Gadd45a can complex with the two other members of the Gad d45 family of stress-induced proteins, human Gadd45b (MyD118) and Gadd45g ( CR6). Gel-exclusion chromatography, native gel electrophoretic analysis, en zyme-linked immunosorbent assay, and chemical cross-linking showed that rec ombinant Gadd45a forms dimeric, trimeric, and tetrameric species in vitro, the dimers being the predominant form. Deletion mutant and peptide scanning analyses suggest that Gadd45a has two self-association sites: within N-ter minal amino acids 33-61 and within 40 C-terminal amino acids. Despite the l ow abundance of Gadd45a in the cell, oligomer-forming concentrations can pr obably be achieved in the foci-like nuclear structures formed by the protei n upon overexpression. Evidence for a potential role of Gadd45a self-associ ation in altering DNA accessibility on damaged nucleosomes is presented.