The Fanconi Anemia Protein, FANCG, Binds Directly to the SH3 Domain of aII Spectrin and This Interaction May Play An Important Role in the Stability of aII Spectrin and Its Function in Repair of DNA Interstrand Cross-Links

Fanconi anemia (FA) is a genetic disorder characterized by bone marrow failure, a predisposition to cancer, congenital abnormalities and a cellular hypersensitivity to DNA interstrand cross-linking agents. This hypersensitivity in FA cells correlates with a defect in ability to repair interstrand cross-links. We have shown that the structural protein, nonerythroid a spectrin (aIISp), plays an important role in the repair of DNA interstrand cross-links and that there is a deficiency in aIISp in FA cells. The reduced levels of aIISp in FA cells are due to reduced stability of this protein. We propose that the stability of aIISp is dependent upon one or more of the FA proteins. The present study was undertaken in order to get a clearer understanding of the proposed role of FA proteins in maintaining the stability of aIISp in the cell and the functional importance of this relationship in the repair of DNA interstrand cross-links and the repair defect in FA cells. For these studies, FA proteins were examined for their ability to directly interact with aIISp and, if so, to map the sites of interaction. Four overlapping regions of aIISp were constructed and binding of FA proteins to each of these regions was examined using yeast two-hybrid analysis. Of the proteins examined, FANCG was found to interact with one of these regions of aIISp and specifically with the SH3 domain within this region. The site of interaction in FANCG was mapped to a SH3 domain binding motif, which contains a consensus sequence with preference for the SH3 domain of aIISp. The sites of interaction in both FANCG and aIISp were confirmed using site-directed mutagenesis. Two FA proteins that did not contain any SH3 binding motifs, FANCC and FANCF, did not interact with the SH3 domain of aIISp or any of the other regions of aIISp. These results thus demonstrate that one of the FA proteins, FANCG, contains a class of motifs that has specificity for binding to SH3 domains and binds to the SH3 domain of aIISp via this motif. This binding is important in the DNA repair process and in the FA repair defect, as is shown by our in vivo studies in which FA-G cells, transformed with the FANCG cDNA and stably expressing FANCG, showed normal levels of aIISp and a correction of the defect in ability to repair DNA interstrand cross-links. We therefore propose that the binding of FANCG to aIISp is not only important for the stability of aIISp in cells but also for the role aIISp plays in the DNA repair process. Thus a deficiency in FANCG, such as occurs in FA-G cells, could lead to reduced stability of aIISp which in turn could be an important factor in the defective DNA repair pathway in FA cells.