Determining the molecular mechanisms of ribosomal export through the nuclear pore.

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The long-term objective of our research program is to study the maturation pathways of essential RNA-protein complexes in different organisms. Ribosomes are the cellular protein production machinery and as such vital to every cell and organism. Ribosome biogenesis is a complex process that involves more than 200 proteins. In cells, ribosomes are processed and assembled within the nucleolus, a specialized compartment of the nucleus, and, once mature, they are exported to the cytoplasm to function during translation. Molecules transporting from the nucleus to the cytoplasm pass through the nuclear pore, a macromolecular complex embedded in the nuclear membrane. Specified proteins, export factors, that are particular to different transporting complexes, mediate transport through the pores. While transport factors have been identified for ribosomes, very little is known about the actual mechanisms of ribosome export. In particular, it is still unclear how a macromolecule as big as the ribosome (~35nm diameter) can pass through the top (~25nm diameter) or spokes (~12.5nm diameter) of the nuclear basket, a nuclear structure of the nuclear pore controlling access to the transport channel, which is believed to be universal to all nuclear pores in all organisms. We want to understand the molecular mechanisms of ribosome export through the nuclear pore complex, and the role of the nuclear basket as well as the function of specific nucleoporins during the targeting, translocation and finally release of ribosomes into the cytoplasm. In the baker's yeast S.cerevisiae, basket-less pores have been identified in an area adjacent to the nucleolus. This suggests the possibility that these basket-less pores could be dedicated to ribosome export. However, basket-less pores have not yet been observed in human cells, which raises the question of how evolutionary conserved ribosome maturation pathways are. Due to their vital function, ribosomes are highly conserved between species, even organisms as distant as Archeabacteria and humans, and so are most aspects of their maturation. However, there may be significant differences due to the complexity of the organism, and the potential consequences of dysfunctional ribosomes, such as malignant cell proliferation and disease development in humans. We will establish the role of the nuclear basket in ribosome export and the function of basket-less nuclear pores, which is as yet unclear. Furthermore, we will determine the existence of basket-less pores in human cells, and examine the differences in ribosome export between lower and higher eukaryotes, namely yeast and humans.**