Uncovering Novel Roles of Tandem BRCA1 C‐terminal (BRCT) Domains in DNA Repair, Genome Stability and Breast Cancer

Abstract

Pax transactivation domain‐interacting protein (PAXIP1/PTIP) has an important role in preserving genomic stability and is essential for cell survival following DNA damage, although the precise mechanism is undefined. PTIP is primarily a nuclear protein consisting of three pairs of tandem BRCA1 C‐terminal (tBRCT) domains. These BRCT domains are required for its assembly into multiple protein complexes. It is known that mutations that disrupt BRCT domain binding correlate with increased cancer rates. To determine its precise role in DNA repair, we profiled the expression of PTIP in breast cancer cell lines and observed differences in protein localization. We have recombinantly expressed the C‐terminal BRCT domains of PTIP, (BRCT36), which retains the ability to form nuclear foci. Using this construct, we set out to investigate how PTIP assembles into DNA repair complexes by conducting differential and ultra‐centrifugation experiments. The biophysical characterization of PTIP uncovers new oligomerization interfaces of higher order structures composed exclusively of BRCT36. These studies indicate that BRCT36 participates in the formation of multiple structurally and stoichiometrically diverse complexes during DNA repair.These studies suggest new and unusual functional roles of PTIP in DNA repair, genome stability and breast cancer.