Understanding the Functional Role of the C-Terminal Domain of the Conserved Protein LepA
A large number of biochemical reactions that enable and regulate life are mediated by enzymatic proteins, which work by lowering the activation barrier of a biochemical reaction to increase the rate at which the reaction occurs. During translation, the ribosome is assisted by multiple different proteins which efficiently catalyze the formation polypeptides, to achieve a functional protein with as little error as possible. Many of the ribosomal associated proteins are GTPases, such as the Elongation Factors (EF-Tu and EF-G), which act as molecular switches by catalyzing the hydrolysis of GTP into GDP and an inorganic phosphate. For example, EF-G catalyzes the translocation of peptidyl-tRNA from the aminoacyl site, to the peptidyl site, as well as the movement of mRNA by one codon.
There are many translational GTPases whose functions are still unknown, one of them being LepA. LepA is a highly conserved GTPase, present in bacteria and eukaryotic mitochondria and chloroplasts. It is the third most highly conserved protein in bacteria, falling behind EF-Tu and EF-G, implying that it is likely serves a significant function in the cell. LepA is very similar in structure EF-G, apart from it’s unique C-terminal domain (CTD). The role of the CTD and how it makes LepA different from EF-G in function are still unknown and is the basis for this study.
For my project, six variants of LepA were constructed to study the role of the CTD including C-terminal truncations and single point mutations. Initial data from our lab showed that several of these variants had altered GTP hydrolysis rates presumably because key functional amino acids in the CTD were removed or mutated. To ensure this was the case, several control experiments were carried out including measuring the binding affinity of LepA wild type and variants to nucleotides (GTP / GDP) were measured via rapid kinetics using a stopped flow. To do so, a fluorescent analog of GTP / GDP was used to monitor binding and unbinding to LepA. The results showed that all LepA variants tested were able to bind nucleotides with the same affinity, suggesting that altered nucleotide binding was not responsible for the change in GTP hydrolysis observed prior, and also that LepA has approximately 8 times higher affinity toward GDP when compared to GTP. Overall the work has contributed to the understanding of the CTD in LepA, and thereby helping to determine the functional role of LepA in protein synthesis.