SUMO isoforms and conjugation-independent function in DNA double-strand break repair pathways [Cell Biology]

June 25th, 2014 by Hu, Y., Parvin, J. D.

SUMO proteins act in DNA double-strand break (DSB) repair but the pathway specificity of the three major isoforms has not been defined. In experiments in which we depleted the endogenous SUMO protein by RNAi, we found that SUMO1 functioned in all sub-pathways of either homologous recombination (HR) or non-homologous end-joining (NHEJ), whereas SUMO2/3 was required for the major NHEJ pathway, called conservative-NHEJ, but dispensable in other DSB repair pathways. To our surprise, we found that depletion of UBC9, the unique SUMO E2 enzyme, had no effect in HR or alternative-NHEJ (Alt-NHEJ) but was required for conservative-NHEJ. Consistent with this result, both non-conjugatable mutant and wild-type SUMO1 proteins functioned similarly in HR and Alt-NHEJ. These results detail the functional roles of specific SUMO isoforms in DSB repair in mammalian cells, and reveal that SUMO1 functions in HR or Alt-NHEJ as a free protein and not as a protein conjugate.