STRUCTURAL ASPECTS OF PROTEIN SYNTHESIS

Anders Liljas

Molecular Biophysics, Center for Chemistry and Chemical Engineering, Lund University

The structural understanding of the molecular system participating in protein synthesis is currently undergoing rapid changes. From having had essentially only the tRNA:s at atomic resolution and the whole rest of the components dimly seen with the aid of electron microscopy or other low resolution methods we now have great improvements on all frontiers. A brief review of the field will be presented.

The ribosomes alone or in complexes with tRNA:s and factors is currently being investigated with cryo-electron microscopy. Eventhough the resolution remains low the details and their constraints on possible mechanisms is making significant changes to the field. The crystallography on ribosomal subunits looks more and more promising with several groups participating and new methods for phase angle determination employed.

The work on ribosomal components is also moving rapidly ahead. Here NMR and crystallography are the methods of choice. Fragments of rRNA have been difficult to crystallize, probably due to our poor understanding of what determines the tertiary structure of RNA molecules. Here the NMR approach has been more successful in getting detailed structures of different fragments of rRNA. One interesting feature is the unusual types of basepairing that occur in the frequent bulge loops and make the helices continue beyond what was initially expected. Part of the ribosomal decoding site on the 16S RNA is thus determined as well as the so called -sarcin/ricin loop of 23S RNA which is part of the binding site for elongation factors. A rapidly increasing number of structures of ribosomal proteins is also becoming available by NMR and crystallography. Here it is evident that some structural motifs are recurring. Some of these are also found in other RNA binding proteins. Another feature that is repeatedly found is longer or shorter loops usually stabilized by hydrogen bonds into a -ribbon. In some RNA binding proteins such ribbons are found to bind to the RNA. A common feature between ribosomal proteins and other RNA-binding proteins is also the exposed location of aromatic residues on the surface of these proteins. They have been found to interact with the bases of the RNA. The work on determining structures of ribosomal proteins in complex with their binding fragments of the rRNA is in progress.

The last few years has also seen dramatic progress with regard to structures of translation factors. Here the most remarkable progress is on the elongation factors. The structure of EF-G and its mimicry of the ternary complex of EF-Tu with tRNA and a GTP analogue has significant functional implications.