The Active 80S Ribosome-Sec61 Complex

The atomic models of the ribosomal subunits, togetherwith structural investigations of functional ribosomalcomplexes by X-ray crystallography and 3D cryo-EM,have led to a basic understanding of the structure and dynamics of protein biosynthesis in prokaryotic systems. Incontrast, our knowledge about eukaryotic ribosomes issparse. The evolutionary conservation of rRNA and ribosomal proteins suggests that the ribosomal subunits sharea similar spatial arrangement and that the fundamentalmechanism of protein biosynthesis is the same. However,the actual degree of similarity is not known, and significant differences are known to exist. Due to insertion elements in the rRNA molecules (Gerbi 1996) and the presence of 20 to 30 additional proteins (Wittmann-Liebold1986; Wool et al. 1990; Planta and Mager 1998), eukaryotic ribosomes are larger than their prokaryotic counterparts. There are functional differences, e.g., in the mechanism of initiation (Sachs et al. 1997), or in themechanism of elongation, as demonstrated by EF3, athird elongation factor in yeast (Triana-Alonso et al.1995). Additionally, several antibiotics are specific forone class of ribosomes or the other (Spahn and Prescott1996). Furthermore, the ribosome has to play a role inother biological processes in the cell such as protein folding (Hardesty et al. 2000) and protein transport (Beckmann et al. 1997, 2001; Menetret et al. 2000). Bindingand recognition events in these processes do not necessarily take place at those ribosomal sites that are highlyconserved among bacterial and eukaryotic ribosomes.These and other differences in function are likely to bereflected by differences in structure...