¹Department of Biochemistry and NCBES, National University of Ireland-Galway, Galway, Ireland

Correspondence: Dr CG Morrison, Department of Biochemistry and NCBES, National University of Ireland-Galway, University Road, Galway, Connacht, Ireland. E-mail: ciaran.morrison@nuigalway.ie

Received 14 August 2006; Revised 12 April 2007; Accepted 16 May 2007; Published online 25 June 2007.

Abstract

Microcephalin (MCPH1/BRIT1) forms ionizing radiation-induced nuclear foci (IRIF) and is required for DNA damage-responsive S and G₂-M-phase checkpoints. MCPH1 contains three BRCT domains. Here we report the cloning of chicken Mcph1 (cMcph1) and functional analysis of its individual BRCT domains. Full-length cMcph1 localized to centrosomes throughout the cell cycle and formed IRIF that colocalized with -H2AX. The tandem C-terminal BRCT2 and BRCT3 domains of cMcph1 were necessary for IRIF formation, while the N-terminal BRCT1 was required for centrosomal localization in irradiated cells. Centrosomal targeting of cMcph1 was independent of ATM, Brca1 or Chk1. cMcph1 formed IRIF in ATM- and Brca1-deficient cells, but not in H2AX-deficient cells. Inability to form cMcph1 IRIF impaired the cellular response to DNA damage. These results suggest that the role of microcephalin in the vertebrate DNA damage response is controlled by interaction of the C-terminal BRCT domains with -H2AX.