(a) Expression of unfused wt LysRS, LysRS(K130A), and LysRS(T133A) at 37°C. These proteins have hexa-histidine tag at their C-termini. Mutants were constructed using PCR overlapping mutagenesis. (b) The effects of point mutations on the solubility of LysRS fusion proteins in vivo. Three passenger proteins GNB2L1, ANGPTL4 and FAM3D were fused to the C-termini of wt LysRS, LysRS(K130A), and LysRS(T133A). The expression temperature was 37°C (30°C in case of FAM3D fusion proteins). The representative SDS-PAGE data are shown in left panel. The solubility of fusion proteins obtained by three independent experiments is summarized in right panel. (c) RNA binding analysis of LysRS and its mutants using gel-retardation assay. The binding affinity of wt LysRS, LysRS(K130A), and LysRS(T133A) to 5′-32P-labeled tRNALys was analyzed by gel-retardation assay as described in . For the competition assay, the cold tRNALys (middle) and tRNAPhe (right) of various concentrations (0, 0.46, 1.16, and 2.3 µM) was used. Arrow indicates the LysRS-tRNALys complexes. Note that the relative amounts of tRNALys binding to LysRS, LysRS(K130A), and LysRS(T133A) are 1, 0.35, and 1.17, respectively. (d) The effect of tRNA coexpression on the solubility of LysRS fusion proteins in vivo. GNB2L1 as a C-terminal passenger protein was fused to wt LysRS and LysRS(K130A), and the fusion proteins were expressed at 37°C.