Abstract
The cohesion of sister chromatids in the interval between chromosome replication and anaphase is important for preventing the precocious separation, and hence nondisjunction, of chromatids. Cohesion is accomplished by a ring-shaped protein complex, cohesin; and its release at anaphase occurs when separase cleaves the complex's α-kleisin subunit. Cohesin has additional roles in facilitating DNA damage repair from the sister chromatid and in regulating gene expression. We tested the universality of the present model of cohesion by studying cohesin in the evolutionarily distant protist Tetrahymena thermophila. Localization of tagged cohesin components Smc1p and Rec8p (the α-kleisin) showed that cohesin is abundant in mitotic and meiotic nuclei. RNAi knockdown experiments demonstrated that cohesin is crucial for normal chromosome segregation and meiotic DSB repair. Unexpectedly, cohesin does not detach from chromosome arms in anaphase, yet chromosome segregation depends on the activity of separase (Esp1p). When Esp1p is depleted by RNAi, chromosomes become polytenic as they undergo multiple rounds of replication, but fail to separate. The cohesion of such bundles of numerous chromatids suggests that chromatids may be connected by factors in addition to topological linkage by cohesin rings. Although cohesin is not detected in transcriptionally active somatic nuclei, its loss causes a slight defect in their amitotic division. Notably, Tetrahymena uses a single version of α-kleisin for both mitosis and meiosis. Therefore, we propose that the differentiation of mitotic and meiotic cohesins found in most other model systems is not due to the need of a specialized meiotic cohesin, but due to additional roles of mitotic cohesin.
Highlights
Cohesin is a ring-shaped protein complex which holds sister chromatids together to prevent their untimely separation prior to anaphase
The cohesion of sister chromatids in the interval between DNA replication and mitotic anaphase is important for preventing the precocious separation, and nondisjunction, of chromatids
Cohesion is accomplished by a ring-shaped protein complex, cohesin; and a popular model of cohesion holds that sister chromatids are encircled by cohesin rings and separate upon opening of the rings
Summary
Cohesin is a ring-shaped protein complex which holds sister chromatids together to prevent their untimely separation prior to anaphase (see [1]). It consists of four core components, Smc and Smc, Scc, and a member of the conserved a-kleisin family of proteins, Mdc1/Rad21/Scc in mitotic cells, or Rec in meiotic cells [1,2]. The resolution of cohesion during anaphase is initiated by the cleavage of the a-kleisin component of the cohesin, the opening of the ring structure, and the disappearance of the cohesin from the chromosomes This allows the mitotic separation of the sister chromatids. The fact that ring opening releases the chromatids, together with other evidence (see [1]), led to the popular model that cohesin works by the enclosure of the two sisters by a single ring
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