A novel SMC protein complex in Schizosaccharomyces pombe contains the Rad18 DNA repair protein

In Schizosaccharomyces pombe, rad18 is an essential gene involved in the re pair of DNA damage produced by ionizing radiation and in tolerance of UV-in duced DNA damage. The Rad18 protein is a member of the SMC (structural main tenance of chromosomes) superfamily, and we show that, like the other SMC p roteins in condensin and cohesin, Rad18 is a component of a high-molecular- weight complex. This complex contains at least six other proteins, the larg est of which is Spr18, a novel SMC family member closely related to Rad18, and likely to be its heterodimeric partner. SMC proteins have ATP-binding d omains at the N- and C-termini, and two extended coiled-coil domains separa ted by a hinge in the middle. We show that the N-terminal ATP-binding domai n of Rad18 is essential for all functions, and overexpression of an N-termi nal mutant has a dominant-negative effect. We have identified an important mutation (S1045A) near the C-terminus of Rad18 that separates its repair an d essential roles. Potential models for the role of the Rad18-Spr18 complex during DNA repair are discussed.