Fission Yeast Homologs of Human CENP-B Have Redundant Functions Affecting Cell Growth and Chromosome Segregation

Abstract

Two functionally important DNA sequence elements in centromeres of the fission yeast Schizosaccharomyces pombe are the centromeric central core and the K-type repeat. Both of these DNA elements show internal functional redundancy that is not correlated with a conserved DNA sequence. Specific, but degenerate, sequences in these elements are bound in vitro by the S. pombe DNA-binding proteins Abp1p (also called Cbp1p) and Cbhp, which are related to the mammalian centromere DNA-binding protein CENP-B. In this study, we determined that Abp1p binds to at least one of its target sequences within S. pombe centromere II central core (cc2) DNA with an affinity (K s ‫؍‬ 7 ؋ 10 9 M ؊1) higher than those of other known centromere DNA-binding proteins for their cognate targets. In vivo, epitope-tagged Cbhp associated with centromeric K repeat chromatin, as well as with noncentromeric regions. Like abp1 ؉ /cbp1 ؉ , we found that cbh ؉ is not essential in fission yeast, but a strain carrying deletions of both genes (⌬abp1 ⌬cbh) is extremely compromised in growth rate and morphology and missegregates chromosomes at very high frequency. The synergism between the two null mutations suggests that these proteins perform redundant functions in S. pombe chromosome segregation. In vitro assays with cell extracts with these proteins depleted allowed the specific assignments of several binding sites for them within cc2 and the K-type repeat. Redundancy observed at the centromere DNA level appears to be reflected at the protein level, as no single member of the CENP-B-related protein family is essential for proper chromosome segregation in fission yeast. The relevance of these findings to mammalian centromeres is discussed.The faithful inheritance of a cell's genetic blueprint depends on flawless chromosome segregation at each cell division. Mistakes that result in gain or loss of chromosomes (aneuploidy) are associated with death and disease in humans. Aneuploidy is estimated to be the cause of 25% of spontaneous abortions, is the leading cause of mental retardation, and is associated with certain types of cancers, including most colorectal tumors (9,26,36). Proper segregation in mitosis and meiosis is governed by the centromere, a multifunctional region on each chromosome that is the site of sister chromatid cohesion, kinetochore assembly, and spindle attachment and is subject to cell cycle surveillance (reviewed in references 1 and 16) and epigenetic regulation (reviewed in references 13 and 34). Despite the functional similarities of centromeres in various organisms, wide variation is seen in the underlying centromeric DNA sequences that nucleate assembly of the kinetochore. Not surprisingly, the centromere appears to be a target of species divergence.With the exception of the budding yeasts, which utilize small "point" centromeres (Ͻ0.2 kb), other organisms as diverse as fission yeast (Schizosaccharomyces pombe), Drosophila, and humans contain large "regional" centromeres (40 to 5,000 kb) that are characterized by the presence of re...

Perspective: Transposable Elements, Parasitic Dna, and Genome Evolution

“…The observed dependence on genetic background in mice may be related to redundancy of function. In support of this hypothesis, in fission yeast, when two redundant homologues of CENP-B were mutated the resulting strains were severely compromised in growth rate and morphology, and they exhibited marked chromosome missegregation (Baum and Clarke 2000). These data suggest that CENP-B, which is clearly related to an ancient and widespread family of TEs, can play an important role in host chromosome segregation.…”

Schizosaccharomyces pombe Histone Acetyltransferase Mst1 (KAT5) Is an Essential Protein Required for Damage Response and Chromosome Segregation

“…The first, Hip1, is a putative histone chaperone most closely related to ScHir1 and human HIRA (Blackwell et al 2004). The second, Cbh1, is one of the three CenpB homologs in fission yeast associated with centromere assembly (Baum and Clarke 2000;Nakagawa et al 2002). We tested all three CenpB homologs for interaction with Mst1.…”

“…2. We isolated Cbh1, which is one of the three CenpB homologs in fission yeast (Baum and Clarke 2000;Nakagawa et al 2002). The CenpB homologs are proposed to affect centromere structure (Nakagawa et al 2002) and, in metazoans, CenpB is involved in recruitment of H3-Cen/CenpA (reviewed in Mellone and Allshire 2003).…”

The Basic Helix-Loop-Helix Factor, HAND2, Functions as a Transcriptional Activator by Binding to E-boxes as a Heterodimer

“…Nucleophosmin/B23 is a nucleolar protein that has been implicated in regulating the cell cycle (26,27). CENP-B is a centromeric protein implicated in chromosome segregation (28). These clones were not tested further.…”