The LARP6 La module from <i>Tetrabaena socialis</i> reveals structural and functional differences from plant and animal LARP6 homologues

This study identified the LARP6 La Module from <i>Tetrabaena socialis</i> (<i>T. socialis</i>), a four-celled green algae, in an effort to better understand the evolution of LARP6 structure and RNA-binding activity in multicellular eukaryotes. Using a combination of sequence alignments, domain boundary screens, and structural modelling, we recombinantly expressed and isolated the <i>Ts</i>LARP6 La Module to > 98% purity for <i>in vitro</i> biochemical characterization. The La Module is stably folded and exerts minimal RNA binding activity against single-stranded homopolymeric RNAs. Surprisingly, it exhibits low micromolar binding affinity for the vertebrate LARP6 cognate ligand, a bulged-stem loop found in the 5'UTR of collagen type I mRNA, but does not bind double-stranded RNAs of similar size. These result suggests that the <i>Ts</i>LARP6 La Module may prefer structured RNA ligands. In contrast, however, the <i>Ts</i>LARP6 La Module does not exhibit the RNA chaperone activity that is observed in vertebrate homologs. Therefore, we conclude that protist LARP6 may have both distinct RNA ligands and binding mechanisms from the previously characterized LARP6 proteins of animals and vascular plants, thus establishing a distinct third class of the LARP6 protein family.