The Road to Ribosomes

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Abstract
The genetic material of eukaryotes is surrounded by a nu- clear envelope, which acts as a barrier to the free exchange of proteins and nucleic acids between the nucleoplasm and cytoplasm. Although the resulting physical separation of transcription and translation provides cells with a level of control of gene expression not available to prokaryotes, it also demands the presence of a transport system to selec- tively move macromolecules between the two compart- ments. Such a system must be capable of discriminating between a wide variety of structurally and functionally dis- tinct molecules and may also respond to differing growth and environmental conditions. The biosynthesis and transport of ribosomes in a rapidly growing cell (such as the yeast Saccharomyces cerevisiae ) presents a dramatic paradigm for the magnitude of the problems faced by the nucleocytoplasmic transport sys- tem. Each mRNA encoding a ribosomal protein must first be transported to the cytoplasm, where it is translated. The resulting ribosomal proteins are then imported into the nucleus and then to the nucleolus, where they associ- ate with newly synthesized ribosomal RNA (rRNA) 1 . The preribosomes, so formed, then undergo a series of compli- cated modifications before they are exported back to the cytoplasm, to perform their function in translation. In a rapidly growing culture of yeast that double their riboso- mal content every 1.5 h, each cell must import at z 150,000 ribosomal proteins per minute across the nuclear envelope (while simultaneously exporting z 4,000 ribosomal sub- units per minute; Warner, 1999). Tremendous recent advances in methodology have al- lowed us to trace a sketchy sequential path through the components and mechanisms of ribosome biogenesis (Kressler et al., 1999). This path begins with the produc- tion of ribosomal proteins, continues via their import into the nucleus and association with nascent rRNA in the nu- cleolus, and heads out towards the nuclear envelope again during the still ill-defined steps of preribosomal subunit maturation. However, until now, the path has had a glar- ing pothole: the proteins specifically mediating ribosomal subunit export have remained elusive. In this issue, insight into this process has been gained from studying the pro- cess of ribosome biogenesis in yeast (Ho et al., 2000a). This finding has identified players in this export process, and has potentially linked the final stages of ribosomal subunit maturation with their nuclear export, underscor- ing the continuum of the ribosome life cycle from its birth and maturation in the nucleus to its functional role in translation in the cytoplasm.