Recruitment, loading, and activation of the Smc5–Smc6 SUMO ligase

Abstract

Duplication of the genome poses one of the most significant threats to genetic integrity, cellular fitness and organismal health. Therefore, numerous mechanisms have evolved that maintain replication fork stability in the face of DNA damage and allow faithful genome duplication. The fission yeast BRCT-domain containing protein Brc1, and its budding yeast orthologue Rtt107, has emerged as a "hub" factor that integrates multiple replication fork protection mechanisms. Notably, the cofactors and pathways through which Brc1, Rtt107 and their human orthologue (PTIP) act appeared largely distinct. This either represents true evolutionary functional divergence, or perhaps an incomplete genetic and biochemical analysis of each protein. In this regard we recently showed that like Rtt107, Brc1 supports key functions of the Smc5-Smc6 complex; including its recruitment into DNA repair foci, chromatin association and SUMO ligase activity. Furthermore, fission yeast lacking the Nse5-Nse6 genome stability factor were found to exhibit defects in Smc5-Smc6 function, similar to but more severe than those in cells lacking Brc1. Here we place these findings in context with the known functions of Brc1, Rtt107 and Smc5-Smc6, present data suggesting a role for acetylation in Smc5-Smc6 chromatin loading, and discuss wider implications for genome stability.

Nse5/6 is a negative regulator of the ATPase activity of the Smc5/6 complex

“…Nse5/6 has been identified as a factor required for recruitment of Smc5/6 to collapsed replication forks and for recruitment and/or retention of the complex at defined chromatin sites, which include highly repetitive sequences such as centrosomes, telomeres and the ribosomal DNA array 51,52 . It has been proposed to function in a manner similar to the Scc2-Scc4 cohesin loader complex 22 , but it is worth noting here that Scc2-Scc4 has been shown to stimulate (rather than inhibit) the ATPase activity of cohesin and is capable of binding directly to DNA 53,54 . Nse5/6 therefore cannot work in exactly the same manner, as we show here that it serves to inhibit ATPase activity and has no intrinsic ability to bind DNA.…”

“…It has been proposed to function in a manner similar to the Scc2-Scc4 cohesin loader complex 22 , but it is worth noting here that Scc2-Scc4 has been shown to stimulate (rather than inhibit) the ATPase activity of cohesin and is capable of binding directly to DNA 53,54 . Nse5/6 therefore cannot work in exactly the same manner, as we show here that it serves to inhibit ATPase activity and has no intrinsic ability to bind DNA.…”

“…Nse5/6 is thought to promote recruitment to, or 'loading' of, the Smc5/6 complex onto chromatin 22,23 in a manner similar to that described for the cohesin 'loader complex' Scc2-Scc4 20 [reviewed in refs: 6,24]. In support of this hypothesis: S. pombe cells lacking Nse5/6 display a drastic reduction in the amount of Smc5/6 associated with chromatin 22,25 ; in S. cerevisiae, hypomorphic mutations of Nse5 lead to reduced levels of Smc5/6 associated with stalled replication forks 26 ; in humans the SLF1-SLF2 complex has been shown to recruit Smc5/6 to collapsed replication forks 21 .…”

Identification of novel biomarkers involved in pulmonary arterial hypertension based on multiple-microarray analysis

“…KIF11 is an evolutionarily conserved microtubule motor protein that functions in centrosome and chromosome dynamics in mitosis, KIF11 silencing induced increases in nuclear areas, micronucleus formation, DNA content and chromosome numbers that may contribute to the pathogenesis of cancer [ 28 , 29 ]. The principal activity of the SMC5/6 complex is the maintenance of nuclear genome stability by resolving complex structures and possibly acting as an antagonist of the cohesin complex TheSMC5/6complex exercise many functions, such as the control of unidirectional rDNA replication, neutralizing toxic DNA intermediates during replication, preventing homologous recombination between nonhomologous sequences [ 30 , 31 ]. Anillin actin-binding protein (ANLN) encodes an actin-binding protein that plays a role in cell growth and migration and regulates actin cytoskeletal dynamics in podocytes [ 32 ].…”

Nse5/6 is a negative regulator of the ATPase activity of the Smc5/6 complex

“…Nse5/6 is thought to promote recruitment to, or ‘loading’ of, the Smc5/6 complex onto chromatin ( 22 , 23 ) in a manner similar to that described for the cohesin ‘loader complex’ Scc2-Scc4 ( 20 ) [reviewed in (6,24)]. In support of this hypothesis: Schizosaccharomyces pombe cells lacking Nse5/6 display a drastic reduction in the amount of Smc5/6 associated with chromatin ( 22 , 25 ); in S. cerevisiae , hypomorphic mutations of Nse5 lead to reduced levels of Smc5/6 associated with stalled replication forks ( 26 ); in humans the SLF1-SLF2 complex has been shown to recruit Smc5/6 to collapsed replication forks ( 21 ).…”