Human centrin-2 plays a key role in centrosome function and stimulates nucleotide excision repair by binding to the xeroderma pigmentosum group C protein. To determine the structure of human centrin-2 and to develop an understanding of molecular interactions between centrin and xeroderma pigmentosum group C protein, we characterized the crystal structure of calcium-loaded full-length centrin-2 complexed with a xeroderma pigmentosum group C peptide. Our structure shows that the carboxyl-terminal domain of centrin-2 binds this peptide and two calcium atoms, whereas the amino-terminal lobe is in a closed conformation positioned distantly by an ordered ␣-helical linker. A stretch of the amino-terminal domain unique to centrins appears disordered. Two xeroderma pigmentosum group C peptides both bound to centrin-2 also interact to form an ␣-helical coiled-coil. The interface between centrin-2 and each peptide is predominantly nonpolar, and key hydrophobic residues of XPC have been identified that lead us to propose a novel binding motif for centrin.Human centrin-2 (HsCen-2) 2 is a Ca 2ϩ -binding protein of the calmodulin-parvalbumin EF-hand superfamily (2, 3). HsCen-2 and two other human centrins (4) are best known for functions outside the nucleus. Centrins have essential roles in the duplication and segregation of microtubule organizing centers (5, 6). Much research has focused on these functions, because abnormal centrosome duplication may lead to chromosomal instability and then cancer, an idea supported by discovery of supernumerary abnormal centrosomes in different human tumor cells (7-10). In addition to its role in centrosome function, HsCen-2 is found as a stabilizing component of xeroderma pigmentosum complement protein C (XPC) and HRad23B complexes (11,12). The XPCcontaining heterotrimer is involved in recognition of DNA lesions and initiation of global genome nucleotide excision repair. Global genome nucleotide excision repair is an important DNA repair pathway for damage caused by UV radiation, carcinogens, and chemotherapeutic agents, and impairment of XPC function is associated with the genetic disorder xeroderma pigmentosum. HsCen-2 appears to promote DNA binding by XPC both in vivo and in vitro and increases the specificity of the heterotrimer for damaged DNA (12, 13). The mechanism by which HsCen-2 binds to XPC is not understood.Centrins are also involved in cilia function (14). Moreover, Ca 2ϩ -triggered assembly of HsCen-1 and transducin appears to regulate transducin translocation through the connecting cilium of vertebrate photoreceptors (15)(16)(17)(18). Whereas the first centrin, also known as caltractin (Cdc31p), was found in fibers linking the flagellar apparatus to the nuclei of Tetraselmis striata green alga (19), homologs have since been identified in many organisms, including fungi, plants, and higher eukaryotes. Functional similarities between centrins from various species seem likely. For example, Ca 2ϩ -induced contractions of a fiber formed by caltractin and Sfi1p are thought to play ...