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Abstract

Protein synthesis in eukaryotes is mediated by both cytoplasmic and membrane-bound ribosomes. During the co-translational translocation of secretory and membrane proteins, eukaryotic ribosomes dock with the protein conducting channel of the endoplasmic reticulum. An understanding of these processes will require the detailed structure of a eukaryotic ribosome. To this end, we have compared the three-dimensional structures of yeast and rabbit ribosomes at 24 Å resolution. In general, we find that the active sites for protein synthesis and translocation have been highly conserved. It is interesting that a channel was visualized in the neck of the small subunit whose entrance is formed by a deep groove. By analogy with the prokaryotic small subunit, this channel may provide a conserved portal through which mRNA is threaded into the decoding center. In addition, both the small and large subunits are built around a dense tubular network. Our analysis further suggests that the nascent chain exit tunnel and the docking surface for the endoplasmic reticulum channel are formed by this network. We surmise that many of these features correspond to rRNA, based on biochemical and structural data. Ribosomal function is critically dependent on the specific association of small and large subunits. Our analysis of eukaryotic ribosomes reveals four conserved inter-subunit bridges with a geometry similar to that found in prokaryotes. In particular, a double-bridge connects the small subunit platform with the interface canyon on the large subunit. Moreover, a novel bridge is formed between the platform and the base of the L1 domain. Finally, size differences between mammalian and yeast large subunit rRNAs have been correlated with five expansion segments that form two large spines and three extended fingers. Overall, we find that expansion segments within the large subunit rRNA have been incorporated at positions distinct from the active sites for protein synthesis and translocation.