Dimerization of CtIP, a BRCA1- and CtBP-interacting Protein, Is Mediated by an N-terminal Coiled-coil Motif*

CtIP is a transcriptional co-regulator that binds a num-ber of proteins involved in cell cycle control and cell de-velopment, such as CtBP (C terminus-binding protein), BRCA1 (breast cancer-associated protein-1), and LMO4 (LIM-only protein-4). The only recognizable structural motifs within CtIP are two putative coiled-coil domains located near the N and C termini of the protein. We now show that the N-terminal coiled coil (residues 45–160), but not the C-terminal coiled coil, mediates homodimeriza-tion of CtIP in mammalian 293T cells. The N-terminal coiled coil did not facilitate binding to LMO4 and BRCA1 proteins in these cells. A protease-resistant domain (resi-dues 27–168) that minimally encompasses the putative N-terminal coiled coil was identified by matrix-assisted laser desorption ionization time-of-flight mass spectrom