RNA-protein complexes.
RNA-binding proteins are an extremely diverse group of proteins, reflecting the diverse functional requirements of cellular RNAs. Whereas the number of structures of RNA-binding proteins or modules is increasing at a reasonable rate, that of protein-RNA complexes increments by only a few each year. The recently determined structure of a complex from the U2 small nuclear ribonucleoprotein particle shows the subtleties of RNA stem-loop recognition by ribonucleoprotein modules. A second structure provides the first direct information on double-stranded RNA recognition by the double-stranded RNA-binding module that occurs in a variety of functionally distinct proteins. Another two new complexes concern proteins interacting with tRNA. The first is methionyl-tRNAf(Met) transformylase, which has to compete with elongation factor Tu for charged initiator tRNAMet and does so by recognising specific features of the acceptor stem of tRNAf(Met). The second is prolyl-tRNA synthetase, complexed with its cognate tRNA, that has to specifically recognise the two guanines common to all tRNA anticodons specific for proline.