Modulation of tRNA(iMet), eIF-2, and eIF-2B Expression Shows that GCN4 Translation Is Inversely Coupled to the Level of eIF-2·GTP·Met-tRNA(iMet) Ternary Complexes

108

Eukaryotic initiation factor (eIF) 2B catalyzes the nucleotide activation of eIF2 to its active GTP-bound state. The exchange activity has been mapped to the C terminus of the eIF2B⑀ subunit. We have determined the crystal structure of residues 544 -704 from yeast eIF2B⑀ at 2.3-Å resolution, and this fragment is an all-helical protein built around the conserved aromatic acidic (AA) boxes also found in eIF4G and eIF5. The eight helices are organized in a manner similar to HEAT repeats. The molecule is highly asymmetric with respect to surface charge and conservation. One area in the N terminus is proposed to be directly involved in catalysis. In agreement with this hypothesis, mutation of glutamate 569 is shown to be lethal. An acidic belt and a second area in the C terminus containing residues from the AA boxes are important for binding to eIF2. Two mutations causing the fatal human genetic disease leukoencephalopathy with vanishing white matter are buried and appear to disrupt the structural integrity of the catalytic domain rather than interfering directly with catalysis or binding of eIF2.

Section: Methodsmentioning

confidence: 99%

Structure of the Catalytic Fragment of Translation Initiation Factor 2B and Identification of a Critically Important Catalytic Residue

Boesen

Mohammad

Pavitt

et al. 2004

108

“…Yeast strains H117 (gcn2-101 gcn3-101 his1-29 ino1 ura3-52 ϽHIS-4-lacZ, URA3Ͼ) and H271 (gcd11-507 gcn2-101 gcn3-101 his1-29 ino1 ura3-52 ϽHIS-4-lacZ, URA3Ͼ) (19) were printed to medium containing 5-flouroorotic acid to isolate the Ura Ϫ derivatives J208 (GCD11 ϩ ) and J210 (gcd11-507), respectively. The high copy number URA3 plasmids carrying SUI2 (p925) and IMT4 (C50) were described previously (20). The gcd11⌬ strain J212 (MAT␣ his3-⌬1 leu2-⌬0 ura3-⌬0 gcd11⌬ ::KanMX p[GCD11, URA3]) was isolated as a haploid segregant from the heterozygous diploid gcd11⌬/ GCD11 strain obtained from the yeast genome deletion project.…”

Section: Methodsmentioning

confidence: 99%

“…The high copy number HIS3 plasmid encoding SUI2 (pC1682) was generated by subcloning a ϳ2.4-kb BamHI fragment from plasmid p1097 (22) to the vector pRS423. The high copy number HIS3 plasmid encoding IMT4 (pC1683) was generated by transferring a ϳ0.8-kb SalI-BamHI fragment from the plasmid p1776 (20) to the plasmid pRS423.…”

Section: Methodsmentioning

confidence: 99%

X-ray Structure of Translation Initiation Factor eIF2γ

Roll-Mecak

Alone

Cao

et al. 2004

The x-ray structure of the ␥-subunit of the heterotrimeric translation initiation factor eIF2 has been determined to 2.4-Å resolution. eIF2 is a GTPase that delivers the initiator Met-tRNA to the P site on the small ribosomal subunit during a rate-limiting initiation step in translation. The structure of eIF2␥ closely resembles that of EF1A⅐GTP, consisting of an N-terminal G domain followed by two ␤-barrels arranged in a closed configuration with domain II packed against the G domain in the vicinity of the Switch regions. The G domain of eIF2␥ has an unusual zinc ribbon motif, not previously found in other GTPases. Structure-based site-directed mutagenesis was used to identify two adjacent features on the surface of eIF2␥ that bind the ␣-subunit and Met-tRNA i Met , respectively. These structural, biochemical, and genetic results provide new insights into eIF2 ternary complex assembly.Translation initiation involves assembly of a large protein-RNA complex at the initiation codon of a mRNA in three main steps: binding of initiator Met-tRNA and mRNA with associated factors to the small ribosomal subunit, pairing of the anticodon of Met-tRNA i Met with the AUG start codon, and joining of the two ribosomal subunits to form the 80 S initiation complex. In eukaryotes, this process requires at least twelve distinct translation initiation factors (eIFs) 1 and hydrolysis of two molecules of GTP (reviewed in Ref. 1). Translation initiation factor eIF2 is a heterotrimeric GTPase that delivers the Met-tRNA i Met to the small ribosomal subunit as part of a ternary complex with GTP. Pairing between the anticodon of the Met-tRNA i Met and the AUG start codon triggers hydrolysis of GTP by eIF2 and eIF2⅐GDP is released, leaving Met-tRNA i Met bound in the P site. eIF2⅐GDP cannot bind Met-tRNA i Met and is converted to eIF2⅐GTP by the heteropentameric exchange fac-eIF2 is a stable complex of three subunits (␣, ␤, and ␥), each essential for viability in yeast. Since their discovery, highly conserved eIF2 subunits have been identified in all eukaryotes (reviewed in Ref.2) and also archaebacteria (3). Biochemical and genetic analyses have demonstrated that the three subunits have distinct activities during translation initiation. The ␥-subunit binds GTP and Met-tRNA i Met (reviewed in Ref. 4). GTP hydrolysis by the eIF2 ternary complex bound to the 40 S ribosomal subunit is stimulated by eIF5, which interacts with eIF2␤ (5). The ␤-subunit also interacts with Met-tRNA i Met and has been reported to bind mRNA (6). Given the pivotal role eIF2 plays in eukaryotic translation initiation, it is not surprising that it represents an important target for regulation. In response to various environmental stressors (viral infection, amino acid starvation, heme deficiency, ER stress etc), the ␣-subunit is phosphorylated on Ser 51 , abolishing translation initiation by preventing eIF2B catalyzed GDP-GTP exchange.The ␥-subunit of eIF2 belongs to the superfamily of GTPbinding proteins and is most closely related to the translation elongation factor...

“…5, 27, 28, and 29). Phosphorylation of the ␣-subunit at a conserved serine residue (Ser-51) converts eIF2 to a competitive inhibitor of eIF2B-catalyzed nucleotide exchange (30,31). This results in functional sequestration of limiting amounts of eIF2B and leads to decreased levels of active eIF2 and global protein synthesis.…”

Section: Eukaryotic Translation Initiation Factor Eif2mentioning

confidence: 99%

Biochemical Analysis of the eIF2βγ Complex Reveals a Structural Function for eIF2α in Catalyzed Nucleotide Exchange

Nika

Rippel

Hannig

2001

Eukaryotic translation initiation factor eIF2 is a heterotrimer that binds and delivers Met-tRNA iMet to the 40 S ribosomal subunit in a GTP-dependent manner. Initiation requires hydrolysis of eIF2-bound GTP, which releases an eIF2⅐GDP complex that is recycled to the GTP form by the nucleotide exchange factor eIF2B. The ␣-subunit of eIF2 plays a critical role in regulating nucleotide exchange via phosphorylation at serine 51, which converts eIF2 into a competitive inhibitor of the eIF2B-catalyzed exchange reaction. We purified a form of eIF2 (eIF2␤␥) completely devoid of the ␣-subunit to further study the role of eIF2␣ in eIF2 function. These studies utilized a yeast strain genetically altered to bypass a deletion of the normally essential eIF2␣ structural gene (SUI2). Removal of the ␣-subunit did not appear to significantly alter binding of guanine nucleotide or Met-tRNA i Met ligands by eIF2 in vitro. Qualitative assays to detect 43 S initiation complex formation and eIF5-dependent GTP hydrolysis revealed no differences between eIF2␤␥ and the wild-type eIF2 heterotrimer. However, steady-state kinetic analysis of eIF2B-catalyzed nucleotide exchange revealed that the absence of the ␣-subunit increased K m for eIF2␤␥⅐GDP by an order of magnitude, with a smaller increase in V max . These data indicate that eIF2␣ is required for structural interactions between eIF2 and eIF2B that promote wild-type rates of nucleotide exchange. We suggest that this function contributes to the ability of the ␣-subunit to control the rate of nucleotide exchange through reversible phosphorylation. Eukaryotic translation initiation factor eIF21 is a heterotrimeric GTP-binding protein that in yeast is encoded by the essential genes SUI2, SUI3, and GCD11. eIF2 binds charged initiator tRNA (Met-tRNA i Met ) in a GTP-dependent manner to form a stable ternary complex that interacts with the small ribosomal subunit and additional initiation factors (including eIF3 and eIF1A) to form a 43 S initiation complex (reviewed in Refs. 1 and 2-5). This complex binds at or near the 5Ј-end of capped mRNAs and "scans" the mRNA in a 5Ј to 3Ј direction until an AUG start codon in proper context is encountered. Genetic analyses in yeast have demonstrated a function for eIF2 in the selection of the translational start site (6, 7). Recognition of the start codon is accompanied by eIF5-mediated hydrolysis of eIF2-bound GTP, which facilitates release of an inactive eIF2⅐GDP binary complex as well as several other initiation factors. The 40 S/mRNA/Met-tRNA iMet complex interacts with a 60 S ribosomal subunit to form an 80 S initiation complex that can then enter the elongation phase of protein synthesis. Conversion of the inactive eIF2⅐GDP binary complex to eIF2⅐GTP, which is capable of rebinding initiator tRNA, is catalyzed by the nucleotide exchange factor eIF2B.To execute its functions in translation initiation, eIF2 must interact with multiple ligands as well as other components of the translational apparatus. The former include guanine nucleotides and initi...