Title: Structural Studies of the Fungal pre-mRNA 3'-end Processing Machinery
Description: During mRNA synthesis, pre-mRNAs must be cleaved and polyadenylated at their 3'-end to be fully mature, before being exported from the nucleus. In yeast, there is a large protein machinery comprised of dozens of proteins that work together to perform these two reactions. Some of these proteins are capable of recognizing and binding key sequence elements in the pre-mRNA, effectively directing where in the transcript the cleavage and polyadenylation occur. In this thesis, recently reported structural findings related to the pre-mRNA 3'-end processing machinery are summarized. Within this machinery, the Cleavage Factor IA (CF-IA) complex is comprised of the Rna14, Rna15, and Pcf11 and Clp1 proteins. Results reported here include the crystal structure of the Rna14-Rna15 complex, which indicates that the Rna14 protein forms a dimer that has inherent conformational variability. The Rna15 protein binds to the C-terminal domain of Rna14, and is connected to the Rna14 HAT domain by a flexible linker, which may indicate that Rna15 functions somewhat independently of the Rna14 HAT domain.

The complete CF-IA complex is explored in detail, including protein-protein interactions within the complex and the stoichiometric ratios of CF-IA components. Unlike previous reports, results indicate that CF-IA may form a dimer with a 2:2:2:2 stoichiometry of Rna14:Rna15:Clp1:Pcf11. Also reported are projects unrelated to CF-IA, including the crystal structure of the biotin-dependent alpha(6)beta(6) geranyl-CoA carboxylase (GCC) holoenzyme. Comparison of GCC to the closely related 3-methylcrotonyl CoA carboxylase (MCC) holoenzyme reveals a conserved domain swap in the carboxyltransferase (CT) domains of both enzymes. This domain swap is not present in the related biotin-dependent carboxylases propionyl-CoA carboxylase (PCC) and acetyl-CoA carboxylase (ACC), which may indicate a distinct lineage for biotin-dependent carboxylases that target the γ-carbon. In addition, comparison of the two structures also reveals a conserved Phe191 in MCC that is absent in GCC. Phe191 blocks a key substrate-binding pocket and explains the differences in substrate-specificities between MCC and GCC.

The role of Phe191 is tested by site-directed mutagenesis to a Glycine to open the pocket in MCC and by mutating a structurally equivalent Glycine to Phe to close the pocket in GCC. These mutations can convert MCC to a GCC and vice versa.