Characterizing the CRD-BP-RNA interaction in-vitro and in cells.

The highly conserved family of RNA-binding proteins known as the VICKZ RNA-binding proteins play an integral role in the formation of cytoplasmic RNPs which leads to the stabilization, localization and translational control of many mRNA transcripts in the cell. The key investigation of this thesis was to analyse the binding ability of the VICKZ protein family member, the coding region determinant-binding protein (CRD-BP), both in-vitro and in cells. CRD-BP has four K-homology (KH) domains and two RNA-recognition motif (RRM) domains. Deletion studies in CRD-BP orthologs have shown that the KH domains, and not the RRM domains, are predominantly responsible for binding to RNA substrates. However, it is still unclear to what extent each of the KH domains play in their physical interaction with RNA molecules, nor is it known if each of the KH domains an play equal role in interacting with different RNA substrates. In an effort to address the above questions, we used site-directed mutagenesis to mutate the first glycine of the G-X-X-G motif in each KH domain separately, and in combinations. We mutated the glycine to an aspartate to introduce both physical and electrostatic hindrance for binding at the G-X-X-G motif. The goal was to determine if such a mutation can disrupt CRD-BP's ability to bind its RNA substrates both in-vitro and in cells. Our results showed that KG single mutants KH2, KH3 and KH4 did not disrupt the CRD-BP-c-myc CRD RNA interaction in-vitro. CRD-BP KH1 single mutant exhibited a modest reduction in binding to the c-myc CRD RNA substrate in-vitro. However, double KH domain mutations (KH1-2, KH1-3, and KH2-4) resulted in a complete abrogation of CRD-BP's ability to bind the c-myc CRD RNA substrate, suggesting these KH domains work in tandem to bind to the c-myc CRD RNA substrate in-vitro. Interestingly, the CRD-BP KH domain double mutant, KH3-4, showed only a modest reduction in the c-myc CRD RNA substrate binding, suggesting that the first glycine in the G-X-X-G motif of KH3 and KH4 doe