ABSTRACT
The translation initiation factor aIF2 belongs to the aIF-2-beta/aIF-5 family which is active in the early steps of protein synthesis by forming a ternary complex with GTP and the initiator tRNA. It is involved in the delivery of Met-tRNAiMet to the 40S ribosomal subunit. The solution structure of the intact b-subunit of aIF2 from Sulfolobus solfataricus has been solved by 1H NMR and results composed of an unfolded N-terminus and a folded core domain with four a-helices and three b-strands. The comparison of this structure with the protein of the same family, that we reported here, suggested that that Zn ions can be useful for the correct folding of the C-terminus portion. Starting from this evidence, here we also report a structural homology calculation. The comparison between the Zn-free and Zn-bound forms suggests a possible structure of the terminal region b-subunit of aIF2 from S. solfataricus in presence of Zn. Synchrotron Radiation and Structural Proteomics Edited by Eugenia Pechkova and Christian Riekel Copyright © 2012 Pan Stanford Publishing Pte. Ltd. www.panstanford.com
13.1 INTRODUCTIONThe translation initiation factor aIF2 is a heterotrimeric protein, consisting of a-, b-, and g-subunits, with high sequence similarity among proteins of the same family.It plays a critical role in the initiation of protein synthesis by forming a ternary complex with GTP (guanosine-5’-triphosphate) and the aminoacylated initiator methionyl-tRNA (Met-tRNAiMet). This complex binds to the small ribosomal subunit (Bell and Jackson, 1998), and with the aid of other translation factors scans mRNA from the 5’ end. Upon recognition of the initiation codon, GTP is hydrolyzed and the eIF2-GDP (guanosine diphosphate) complex is released. This leads to the assembly of the 80S ribosome at the initiation codon and to the start of protein elongation. The recycling of eIF2 between successive rounds of translation requires an additional protein factor, the guanine nucleotide exchange factor IF2B, which catalyzes the exchange of GDP bound to eIF2 for GTP (Kimball, 1999; Pestova and Hellen, 2000).Distinct functions have been observed for each subunit of eIF2. The β-subunit is a global regulator of protein synthesis in eukaryotes. Phosphorylation of eIF2a regulates the exchange rate of GDP to GTP in a/eIF2, altering its availability to initiate the translation through the inhibition of Met-tRNAiMet binding (Pain, 1996). The β-subunit is responsible for GTP binding, and its similarity to EF-Tu (~27% identity, ~50% similarity) allowed the identification of the Met-tRNAiMet binding region (Schmitt et al., 2002). The β-subunit of eIF2 is involved in a variety of interactions with other translation factors. For example, its N-terminus binds to eIF5, the GTPase activating factor for eIF2, and to the β-subunit of the exchange factor eIF2b (Asano et al., 1999). This region has also been shown to bind RNA in vitro through three lysine repeats (Laurino et al., 1999). The C-terminal region of eIF2β contains another potential RNA binding motif. Mutations in this C2-C2 zinc finger result in a spontaneous GTPase activity and alter the correct recognition of the AUG codon (Huang et al., 1997). The β-subunit has also been involved in the binding to the β-subunit of eIF2B and to crosslink GTP and Met-tRNAiMet (Bommer and Kurzchalia, 1989; Gaspar et al., 1994).
