Structural and Biochemical Characterization of Domains in the Posttranscriptional Regulator LARP6
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The La-related protein 6 (LARP6) is an RNA binding protein that is involved in the regulation of Type-I collagen biosynthesis. Though the mechanism of binding remains to be determined, it is known this protein binds to a stem loop structure found in the 5’ untranslated region of mRNAs that codes for Type-I collagen through a highly conserved binding domain known as the La Module. The LARP6 protein also contains an uncharacterized LSA domain which contains a well-conserved cysteine. The goal of this study was to characterize the structural significance and contribution to biochemical activity of the conserved domains found in the LARP6 protein. Isolated La Module domain constructs from human, zebrafish, and platyfish LARP6 proteins were used to determine the domain boundaries of each species. Limited proteolysis studies were used to determine the global stability of the isolated La Modules as well as suitability for future structural characterization. RNA binding activity of the three isolated La Modules from each species were measured against the stem loop structure present in the human COL1a1 mRNA using electrophoretic mobility shift assays. The conserved cysteines found in the RRM and LSA domains were targeted for serine mutagenesis in the full-length human protein construct, recombinantly expressed, and purified. Size exclusion chromatography of these serine mutant proteins implicate the cysteine in the LSA in an intramolecular disulfide bond.