The spindle checkpoint

Abstract

Prior to sister-chromatid separation, the spindle checkpoint inhibits cell-cycle progression in response to a signal generated by mitotic spindle damage or by chromosomes that have not attached to microtubules. Recent work has shown that the spindle checkpoint inhibits cell-cycle progression by direct binding of components of the spindle checkpoint pathway to components of a specialized ubiquitin-conjugating system that is responsible for triggering sister-chromatid separation.

Introduction

During mitosis, the two daughter cells receive one copy of each chromosome. Sister chromatids attach via a specialized DNA–protein complex, known as the kinetochore, to microtubules emanating from opposite poles of the mitotic spindle. When and only when all kinetochores have bound microtubules, the mitotic spindle proceeds to pull the sister chromatids apart. Cells go to great lengths to ensure that each daughter cell receives one and only one copy of each chromosome. A surveillance mechanism, known as the spindle checkpoint, is essential for ensuring fidelity in chromosome transmission. The checkpoint monitors correct attachment of kinetochores to microtubules and inhibits sister chromatid separation and thus onset of anaphase when a defect is detected 1, 2.

Genetic screens in budding yeast have identified several components of the spindle checkpoint (Table 1). Subsequently, homologs have been identified in higher eukaryotes showing that the spindle-checkpoint pathway is highly conserved (Table 1). Yeast cells carrying a mutation in any of the genes encoding a component of the spindle checkpoint attempt sister chromatid segregation despite mitotic spindle defects or improper attachment of kinetochores to microtubules, the result of which is disastrous, with cells either losing or acquiring extra chromosomes. Loss of chromosomes in haploid cells leads to cell death. In diploid cells, it can either cause cell death or the accumulation of cancer-causing mutations.

The spindle checkpoint halts cell-cycle progression prior to sister-chromatid separation. Sister-chromatid separation is triggered by ubiquitin-mediated degradation of regulators of sister-chromatid cohesion. A multisubunit ubiquitin ligase known as the cyclosome or anaphase promoting complex (APC [3]) ubiquitinates regulators of sister-chromatid segregation: Pds1 in budding yeast and Cut2 in fission yeast, leading to their destruction by the proteasome. APC-dependent proteolysis is not only important for sister-chromatid separation at the metaphase–anaphase transition but also for a second step in mitosis. APC-dependent proteolysis participates in triggering exit from mitosis by degrading mitotic cyclins, the regulatory subunits of the Cdc2-mitotic kinases [3].

As sister-chromatid separation and exit from mitosis are initiated by APC-dependent proteolysis, it is activation of this degradation machinery or access to its substrates that must be inhibited in order to prevent cell-cycle progression. In the past year, work in Saccharomyces cerevisiae, Schizosaccharomyces pombe, Xenopus and mammalian cell lines has revealed how the spindle-checkpoint pathway prevents progression into anaphase in response to spindle damage. The checkpoint prevents the APC activator Cdc20/Slp1 from activating the ubiquitin ligase activity of the APC. This major breakthrough in our understanding of how the spindle checkpoint impinges on the cell cycle will be the focus of my review.