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Effect of Histidine Rich Sequence Motifs on the RNA Binding Activity of LARP6

La Related Protein 6 (LARP6) is a sub‐family of RNA binding proteins that are members of from the superfamily of La‐Related Proteins (LARPs). In vertebrates, LARP6 is known to bind to the 5′ untranslated region of mRNAs that encode for type I collagens. In contrast, the function of the three LARP6 paralogs in plant LARP6 (LARP6a, LARP6b and LARP6c) is much less characterized. Of the three variations of the LARP6 protein in Arabidopsis thaliana, LARP6a and LARP6c have been shown to bind to poly‐adenosine, poly‐uracil and poly‐cytosine RNA oligonucleotides. This ability to bind a variety of RNA sequences may indicate that plant LARP6 proteins act as translocators, moving mRNAs from the nucleus to the cytoplasm. Unlike LARP6a and LARP6c, LARP6b contains an endogenous histidine‐rich (His‐rich) sequence. His‐rich sequences play diverse roles in biology, including creating a favorable electrostatic surface for intermolecular interactions, directing subcellular localization of proteins, and binding to divalent heavy metals. We hypothesize that the His‐rich sequence in LARP6b may contribute to the RNA binding activity of the protein. Mutant forms of the Arabidopsis LARP6 proteins were constructed using site‐directed mutagenesis, and were expressed and purified from E. coli. Protein structure was analyzed by size exclusion chromatography and limited proteolysis, which provided insight about the structural contributions of the His‐rich sequences. RNA binding assays were carried out with a variety of putative ligands, and demonstrated the effects of deleting the His‐rich sequences in LARP6b. RNA binding was also performed in the presence of select heavy metals known to tightly interact with His‐rich sequences, including Ni2+ and Co2+. These studies provide novel insight into how plants have adapted His‐rich sequences for function in the La‐related protein family of RNA‐binding proteins.