5 The Nucleus and Nucleocytoplasmic Transport in Saccharomyces cerevisiae

Abstract

I. INTRODUCTION
Compartmentation of cell functions is one of the biological features that most clearly differentiates eukaryotic cells from prokaryotic cells. In contrast to the single cytoplasmic compartment of a typical prokaryote, a eukaryotic cell contains numerous membranous organelles that are remarkably distinct both functionally and structurally. The yeast Saccharomyces cerevisiae has served as an important model system for studying organellar structure and function (see Pon and Schatz 1991; Serrano 1991; and other chapters in this volume). A striking merger of molecular genetic, biochemical, and morphological analyses has similarly advanced our understanding of the yeast nucleus, such that studies of yeast are making important contributions to the understanding of this organelle as well.

The structure of the nucleus is classically defined by its morphology. Figures 1 and 2 show views of the yeast nucleus as obtained by electron and fluorescence microscopy, respectively, and illustrate several aspects of interphase nuclear organization. There are at least two functional compartments within the nucleus, one containing the majority of the chromatin (approximately two-thirds by volume; n in Fig. 1; stained red with ethidium bromide in Fig. 2) and one containing the nucleolus (approximately one-third by volume; no in Fig. 1; stained violet with anti-Nop1p antibodies in Fig. 2), where rDNA is transcribed and ribosomes are assembled (Woolford and Warner 1991; Mélèse and Xue 1995). Shown by green fluorescence in Figure 2 is the immunodetection of repressor activator protein 1 (Rap1p) (Shore and Nasmyth 1987), an abundant nuclear protein that binds to the repetitive sequences...