<p>Human Ro ribonucleoproteins (RNPs) are composed of one of the four small Y RNAs and at least two proteins, Ro60 and La. The La protein is a 47 kDa polypeptide that frequently acts as an autoantigen in systemic lupus erythematosus and Sjogren's syndrome [<cite idref="PUB00014753"/>]. In the nucleus, La acts as a RNA polymerase III (RNAP III) transcription factor, while in the cytoplasm, La acts as a translation factor [<cite idref="PUB00014754"/>]. In the nucleus, La binds to the 3'UTR of nascent RNAP III transcripts to assist in folding and maturation [<cite idref="PUB00014760"/>]. In the cytoplasm, La recognises specific classes of mRNAs that contain a 5'-terminal oligopyrimidine (5'TOP) motif known to control protein synthesis [<cite idref="PUB00014755"/>]. The specific recognition is mediated by the N-terminal domain of La, which comprises a La motif and a RNA recognition motif (RRM). The La motif adopts an alpha/beta fold that comprises a winged-helix motif [<cite idref="PUB00014761"/>].</p><p>Homologous La domain-containing proteins have been identified in a wide range of organisms except Archaea, bacteria and viruses [<cite idref="PUB00003341"/>].</p> RNA-binding protein Lupus La