Academic journal article Genetics

Changes in the Nuclear Envelope Environment Affect Spindle Pole Body Duplication in Saccharomyces Cerevisiae

Academic journal article Genetics

Changes in the Nuclear Envelope Environment Affect Spindle Pole Body Duplication in Saccharomyces Cerevisiae

Article excerpt

ABSTRACT

The Saccharomyces cerevisiae nuclear membrane is part of a complex nuclear envelope environment also containing chromatin, integral and peripheral membrane proteins, and large structures such as nuclear pore complexes (NPCs) and the spindle pole body. To study how properties of the nuclear membrane affect nuclear envelope processes, we altered the nuclear membrane by deleting the SPO7 gene. We found that spo7Δ cells were sickened by the mutation of genes coding for spindle pole body components and that spo7Δ was synthetically lethal with mutations in the SUN domain gene MPS3. Mps3p is required for spindle pole body duplication and for a variety of other nuclear envelope processes. In spo7Δ cells, the spindle pole body defect of mps3 mutants was exacerbated, suggesting that nuclear membrane composition affects spindle pole body function. The synthetic lethality between spo7Δ and mps3 mutants was suppressed by deletion of specific nucleoporin genes. In fact, these gene deletions bypassed the requirement for Mps3p entirely, suggesting that under certain conditions spindle pole body duplication can occur via an Mps3p-independent pathway. These data point to an antagonistic relationship between nuclear pore complexes and the spindle pole body. We propose a model whereby nuclear pore complexes either compete with the spindle pole body for insertion into the nuclear membrane or affect spindle pole body duplication by altering the nuclear envelope environment.

THE nuclear envelope is composed of distinct outer and inner nuclear membranes. The outer nuclear membrane is continuous with the endoplasmic reticulum. The inner nuclear membrane is associated with a unique set of proteins, some of which mediate interactions between the nuclear envelope and chromatin (reviewed in Zhao et al. 2009). Nuclear pore complexes traverse both membranes and allow transport of proteins and solutes between the cytoplasm and the nucleus. The inner and outer nuclear membranes fuse in the region surrounding each nuclear pore complex.

In animal cells, the nuclear envelope disassembles as cells enter mitosis and reassembles upon mitotic exit. Nuclear envelope breakdown allows the association of chromosomes with spindle microtubules, which are nucleated from centrosomes that reside in the cytoplasm. In contrast, certain types of fungi, such as the budding yeast Saccharomyces cerevisiae, undergo closed mitosis, where the nuclear envelope remains intact throughout the entire cell cycle. Closed mitosis is possible because the yeast centrosome-equivalent, the spindle pole body (SPB), is embedded in the nuclear envelope, allowing the SPB to nucleate both cytoplasmic and nuclear microtubules.

SPB duplication requires a mechanism for inserting the new SPB into the nuclear envelope (reviewed in Jaspersen and Winey 2004). The new SPB begins to form in late G1/early S phase as satellite material deposited on the cytoplasmic face of an electron-dense region of the nuclear envelope, called the half-bridge. The satellite materialmatures into a duplication plaque, which is then inserted into the nuclear membrane and becomes the daughter SPB. Many genes are known to be required for SPB duplication, and this process has been carefully examined cytologically (Rose and Fink 1987; Winey et al. 1991, 1993; Spang et al. 1995; Bullitt et al. 1997; Adams and Kilmartin 1999; Elliott et al. 1999; Schramm et al. 2000; Jaspersen et al. 2002; Nishikawa et al. 2003; Araki et al. 2006). However, the exact mechanisms by which SPB duplication and insertion occur remain a mystery.

Equally unclear is how nuclear pore complexes are inserted into an intact nuclear envelope (reviewed in Hetzer and Wente 2009). For both the SPB and nuclear pore complexes, the inner and outer nuclear membranes must fuse to allow insertion into the nuclear envelope. Yeast and vertebrate nuclear pore complexes each have four pore membrane (POM) nucleoporins containing transmembrane domains. …

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