Abstract
BackgroundThe SMC proteins are involved in DNA repair, chromosome condensation, and sister chromatid cohesion throughout Eukaryota. Long, anti-parallel coiled coils are a prominent feature of SMC proteins, and are thought to serve as spacer rods to provide an elongated structure and to separate domains. We reported recently that the coiled coils of mammalian condensin (SMC2/4) showed moderate sequence divergence (≈10–15%) consistent with their functioning as spacer rods. The coiled coils of mammalian cohesins (SMC1/3), however, were very highly constrained, with amino acid sequence divergence typically <0.5%. These coiled coils are among the most highly conserved mammalian proteins, suggesting that they make extensive contacts over their entire surface.Methodology/Principal FindingsHere, we broaden our initial analysis of condensin and cohesin to include additional vertebrate and invertebrate organisms and multiple species of yeast. We found that the coiled coils of SMC1/3 are highly constrained in Drosophila and other insects, and more generally across all animal species. However, in yeast they are no more constrained than the coils of SMC2/4 and Ndc80/Nuf2p, suggesting that they are serving primarily as spacer rods.Conclusions/SignificanceSMC1/3 functions for sister chromatid cohesion in all species. Since its coiled coils apparently serve only as spacer rods in yeast, it is likely that this is sufficient for sister chromatid cohesion in all species. This suggests an additional function in animals that constrains the sequence of the coiled coils. Several recent studies have demonstrated that cohesin has a role in gene expression in post-mitotic neurons of Drosophila, and other animal cells. Some variants of human Cornelia de Lange Syndrome involve mutations in human SMC1/3. We suggest that the role of cohesin in gene expression may involve intimate contact of the coiled coils of SMC1/3, and impose the constraint on sequence divergence.
Highlights
The structural maintenance of chromosome (SMC) proteins interact with DNA to carry out several critical functions within the cell including DNA repair, chromosome condensation, and sister chromatid cohesion during mitosis [1,2,3,4,5,6,7,8]
We suggest that the role of cohesin in gene expression may involve intimate contact of the coiled coils of SMC1/3, and impose the constraint on sequence divergence
We found that the coiled coils of SMC1/3 were much more highly conserved than those of SMC2/4 when we compared human sequences to avian, amphibian, and Drosophila orthologs [21]
Summary
The structural maintenance of chromosome (SMC) proteins interact with DNA to carry out several critical functions within the cell including DNA repair, chromosome condensation, and sister chromatid cohesion during mitosis [1,2,3,4,5,6,7,8]. The two SMC subunits of each pair form a dimer, with the hinges associated in the middle and the ATPase domains at the ends of the long coiled-coil rods (Fig. 1). Anti-parallel coiled coils are a prominent feature of SMC proteins, and are thought to serve as spacer rods to provide an elongated structure and to separate domains. We reported recently that the coiled coils of mammalian condensin (SMC2/4) showed moderate sequence divergence (
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