Translation factor GUF1 homolog (also known as elongation factor 4, EF-4, or ribosomal back-translocase) is a chloroplastic ribosome-dependent GTPase in Artemisia annua. It belongs to the LepA subfamily of the TRAFAC class translation factor GTPase superfamily and is classified by PANTHER as subfamily PTHR43512:SF4 (CHLOROPLASTIC). Phylogenetic analysis across multiple plant species, k-mer similarity (>95% identity with chloroplastic isoform A0A2U1PRZ2; 9.6x more similar to Arabidopsis chloroplastic Q9FNM5 than mitochondrial Q9FLE4), and PANTHER classification all confirm this is the chloroplastic GUF1 paralog. The true mitochondrial GUF1 paralog in A. annua is A0A2U1PCN3 (gene CTI12_AA167920). A. annua encodes five EF-4 family members with separate chloroplastic and mitochondrial paralogs. The protein catalyzes GTP hydrolysis upon interaction with chloroplast ribosomes, particularly at the conserved sarcin-ricin loop. GUF1/EF-4 functions as a translation quality control factor that can catalyze reverse (back-)translocation of tRNAs on improperly translocated ribosomes, stabilize specific ribosome conformations, and enhance translation fidelity. It binds to ribosomes in a GTP-dependent manner. The protein contains a five-domain architecture shared with EF-G (domains I, II, III, V) plus a unique C-terminal domain (LepA_C) that makes extensive contacts with ribosome-bound tRNAs. GUF1/EF-4 family members are generally dispensable under optimal growth conditions but become important under stress, promoting faithful protein synthesis under challenging conditions.
Summary: GTPase activity is the core enzymatic function of GUF1/EF-4 family members. The protein hydrolyzes GTP in a ribosome-dependent manner, and the conserved GKS motif in the G-domain nucleotide-binding site is essential for in vivo function. This annotation is well-supported by the domain architecture (IPR000795) and extensive characterization of orthologous proteins.
Reason: GTPase activity is the defining molecular function of the GUF1/EF-4 translation factor family. The protein contains the canonical G-domain with P-loop, switch I, and switch II regions. Direct kinetic measurements in E. coli LepA (PMID:25712150) demonstrate ribosome-dependent multiple turnover GTPase activity, and the conserved catalytic His (His164 in A0A2U1PS28) is essential for this activity.
Summary: GTP binding is intrinsic to the GTPase catalytic cycle of GUF1/EF-4. The protein binds ribosomes in a GTP-dependent manner, and the G-domain contains conserved nucleotide-binding elements including the P-loop and switch regions. This is a well-established property of all GUF1/EF-4 family members.
Reason: GTP binding is a prerequisite for the ribosome-dependent GTPase activity of GUF1. Multiple conserved GTP-binding motifs are present in the G-domain, and GTP-dependent ribosome association has been demonstrated for orthologous EF-4 proteins. Crystal structure at 2.6 A resolution (PMID:27092003) directly visualizes EF-4 bound to a nonhydrolyzable GTP analog on the ribosome.
Summary: Mitochondrial inner membrane localization is INCORRECT for this protein. Phylogenetic analysis across 8 plant species, PANTHER subfamily classification (PTHR43512:SF4, CHLOROPLASTIC), and >95% k-mer identity with chloroplastic isoform A0A2U1PRZ2 demonstrate that A0A2U1PS28 is the chloroplastic GUF1 paralog. The true mitochondrial paralog in A. annua is A0A2U1PCN3. This annotation should be replaced with GO:0009507 (chloroplast).
Reason: OpenScientist deep research revealed a critical organelle mis-assignment. A0A2U1PS28 is classified by PANTHER as PTHR43512:SF4 (CHLOROPLASTIC) and clusters with chloroplastic orthologs across 8 plant species in phylogenetic analysis (distance 0.05 to nearest chloroplastic member vs >0.97 to all mitochondrial members). The Arabidopsis ortholog Q9FNM5 was detected in purified chloroplast preparations by proteomics (PMID:18431481). The mitochondrial annotation was propagated from yeast Guf1 via HAMAP but applies to the wrong paralog.
Summary: Mitochondrial matrix localization is INCORRECT for this protein. This is the chloroplastic GUF1 paralog, not the mitochondrial one. Phylogenetic analysis, PANTHER classification (PTHR43512:SF4, CHLOROPLASTIC), and k-mer similarity all place A0A2U1PS28 in the chloroplastic clade. The correct localization term is GO:0009570 (chloroplast stroma), by analogy with mitochondrial matrix localization of the yeast mitochondrial GUF1.
Reason: The mitochondrial matrix annotation was propagated via HAMAP from yeast Guf1, but A0A2U1PS28 is the chloroplastic paralog. K-mer analysis shows 9.6x higher similarity to Arabidopsis chloroplastic Q9FNM5 than mitochondrial Q9FLE4. The chloroplast stroma is the functional equivalent of the mitochondrial matrix for organellar translation.
Summary: Ribosome binding is a core property of GUF1/EF-4 and essential for its function. The protein binds to ribosomes in a GTP-dependent manner, with extensive contacts mediated by its five-domain architecture including the unique C-terminal domain that interacts with ribosome-bound tRNAs. Structural studies show that EF4 interacts with both PRE-state and POST-state ribosome complexes. Crystal structure at 2.6 A (PMID:27092003) and cryo-EM at 3.8 A (PMID:27137929) directly visualize the ribosome-EF4 complex.
Reason: GUF1/EF-4 is a ribosome-dependent GTPase whose functional substrate is the ribosome itself. Cryo-EM studies reveal extensive contacts between EF-4 domains and ribosomal RNA/tRNAs, and the unique C-terminal domain makes contacts with acceptor stems of A-site and P-site tRNAs. This is a well-characterized property of the entire family. For A0A2U1PS28 (the chloroplastic paralog), this means binding to chloroplast 70S-type ribosomes. No chloroplast-specific ribosome binding GO term currently exists, so GO:0043022 is the most specific available term.
Summary: GUF1/EF-4 promotes organellar protein synthesis, particularly under stress conditions. However, GO:0045727 (positive regulation of translation) is overly broad. Since this protein is the chloroplastic paralog (not mitochondrial), a term like positive regulation of plastid translation would be appropriate, but no such specific term currently exists in GO. Retaining GO:0045727 as a general annotation is acceptable but imprecise.
Reason: The generic positive regulation of translation does not capture the organellar context. GO:0070131 (positive regulation of mitochondrial translation) is incorrect because A0A2U1PS28 is the chloroplastic paralog. No plastid-specific translation regulation term currently exists in GO. Retained as non-core pending availability of a more specific term.
ISS PMID:18442968 The membrane-bound GTPase Guf1 promotes mitochondrial protei...
NEW
Summary: Translation (GO:0006412) is proposed as a new annotation to replace the incorrect GO:0070125 (mitochondrial translational elongation) that was removed. A0A2U1PS28 is the chloroplastic GUF1 paralog and participates in chloroplast/plastid translation, not mitochondrial translation. GO:0006412 is the appropriate general term since GO:0032544 (plastid translation) may not be available in all GO caches. EF-4/GUF1 family proteins are well-characterized translation factors that function in organellar protein synthesis.
Reason: The previously proposed GO:0070125 (mitochondrial translational elongation) was incorrect because A0A2U1PS28 is the chloroplastic paralog, not the mitochondrial one. GO:0006412 (translation) captures the core biological process of this protein without specifying the wrong organellar context. The yeast ortholog Guf1 promotes mitochondrial protein synthesis (PMID:18442968), and by analogy the chloroplastic paralog participates in plastid translation.
A0A2U1PS28 is the chloroplastic paralog (PANTHER PTHR43512:SF4, CHLOROPLASTIC). The true mitochondrial paralog in A. annua is A0A2U1PCN3 (gene CTI12_AA167920), which shares only ~5% k-mer Jaccard similarity with A0A2U1PS28.
Core Functions
GUF1/EF-4 is a ribosome-dependent GTPase that hydrolyzes GTP upon interaction with chloroplast ribosomes. GTP hydrolysis is coupled to conformational changes that catalyze back-translocation of tRNAs on improperly translocated ribosomes or stabilize specific ribosome conformations to enhance translation fidelity. The GTPase activity is triggered by interaction with the ribosomal sarcin-ricin loop. A. annua encodes five EF-4 family members; A0A2U1PS28 is the chloroplastic paralog (PANTHER PTHR43512:SF4, CHLOROPLASTIC), while the mitochondrial paralog is A0A2U1PCN3 (gene CTI12_AA167920).
Promotes mitochondrial protein synthesis. May act as a fidelity factor of the translation reaction, by catalyzing a one-codon backward translocation of tRNAs on improperly translocated ribosomes. Binds to mitochondrial ribosomes in a GTP-dependent manner.
successive removal of the C-terminus impairs ribosome-dependent multiple turnover GTPase activity of EF4, which for the full-length protein is very similar to EF-G
All three GTPase catalytic motifs are perfectly conserved in A0A2U1PS28: P-loop (AHIDHGKS), catalytic switch (DTPGH with essential His164), and G4 specificity box (NKID).
PANTHER classifies A0A2U1PS28 as subfamily PTHR43512:SF4 (CHLOROPLASTIC). K-mer analysis shows 9.6x higher similarity to Arabidopsis chloroplastic Q9FNM5 than mitochondrial Q9FLE4. Phylogenetic clustering places A0A2U1PS28 firmly in the chloroplastic clade across 8 plant species.
GUF1/EF-4 binds GTP as a prerequisite for its ribosome-dependent function. The G-domain contains conserved nucleotide-binding elements (P-loop, switch I, switch II) and the GTP-bound form is required for association with translating chloroplast ribosomes. Crystal structures at 2.6 A resolution directly visualize EF-4 bound to GTP analogs on the ribosome.
we present the crystal structure at 2.6-Å resolution of the Thermus thermophilus 70S ribosome bound to EF-4 with a nonhydrolyzable GTP analog and A-, P-, and E-site tRNAs
A. annua harbors five EF-4 family members, with a chloroplastic paralog (A0A2U1PRZ2) sharing the same gene locus. All three full-length paralogs have identical G-domain motifs and GTP binding is a core function of all of them.
GUF1/EF-4 binds to chloroplast ribosomes carrying tRNAs in the P and A sites, using its five-domain architecture including a unique C-terminal domain (LepA_C) that contacts ribosome-bound tRNA acceptor stems. This ribosome binding is GTP-dependent and central to its function as a translation quality control factor.
The structure reveals the interactions of EF-4 with the A-site tRNA, including contacts between the C-terminal domain (CTD) of EF-4 and the acceptor helical stem of the tRNA
A0A2U1PS28 is the chloroplastic paralog (PANTHER PTHR43512:SF4, CHLOROPLASTIC). The true mitochondrial paralog in A. annua is A0A2U1PCN3 (gene CTI12_AA167920), which shares only ~5% k-mer Jaccard similarity with A0A2U1PS28.
Q: OpenScientist analysis strongly indicates A0A2U1PS28 is the chloroplastic GUF1 paralog based on PANTHER classification, phylogenetic analysis, and k-mer similarity. Can this be confirmed with experimental localization data, e.g. GFP-fusion with chloroplast and mitochondrial markers in plant cells?
Q: Is there a more specific GO molecular function term for translational back-translocase activity that would better capture the unique catalytic mechanism of GUF1/EF-4 compared to other translation-associated GTPases?
Q: Should a GO term for plastid translational elongation be proposed, analogous to GO:0070125 (mitochondrial translational elongation)? Currently GO:0032544 (plastid translation) is the most specific available BP term for chloroplast translation.
Suggested Experiments
Experiment: Express A0A2U1PS28-GFP in A. annua protoplasts or Nicotiana benthamiana leaves and perform confocal microscopy co-stained with MitoTracker and chloroplast autofluorescence to confirm chloroplast targeting and definitively rule out mitochondrial localization.
Hypothesis: Based on PANTHER classification (PTHR43512:SF4, CHLOROPLASTIC), phylogenetic clustering, and >95% k-mer identity with chloroplastic isoform A0A2U1PRZ2, A0A2U1PS28 is predicted to localize to chloroplasts rather than mitochondria.
Experiment: Complement a yeast guf1-delta mutant with A0A2U1PS28 (chloroplastic paralog) vs A0A2U1PCN3 (mitochondrial paralog) and assess mitochondrial translation at optimal and suboptimal temperatures. Compare complementation efficiency to test organelle specificity.
Hypothesis: The mitochondrial paralog A0A2U1PCN3 should rescue yeast guf1 defects more efficiently than the chloroplastic paralog A0A2U1PS28, reflecting organelle-specific adaptation.
Experiment: Characterize the Arabidopsis ortholog At5g08650 (Q9FNM5) T-DNA knockout/knockdown lines for chloroplast translation defects using ribosome profiling of chloroplast mRNAs, especially under cold stress where EF-4 function is most critical.
Hypothesis: Loss of the chloroplastic EF-4 paralog will impair chloroplast translation efficiency, particularly under stress conditions, analogous to the stress-dependent phenotype of yeast guf1-delta for mitochondrial translation.
External Prediction Reviews
These computational predictions are reviewed separately from the GOA annotation set used for this review. The assessments below are from this project and do not constitute official GO annotations or endorsement by GO/UniProt. They are not included in the existing annotation review above.
Prediction method: ProtNLM2 · Version: UniProt 2024_06 pilot
Review rationale: The 661-residue target has the EF4/LepA domain architecture and is assigned to the chloroplastic GUF1 subfamily PTHR43512:SF4 in the cached sequence record. Arabidopsis cpLEPA was experimentally characterized as a chloroplast translation factor (PMID:23166764), providing biological grounding for transfer of this localization within the plant subfamily. This is an inference from subfamily placement, not a targeting assay on Artemisia. The chloroplast term is absent from the cached annotations; the mitochondrial UniRule assignments are a conflicting automated inference rather than decisive localization evidence.
Supporting Evidence:
file:ARTAN/A0A2U1PS28/A0A2U1PS28-uniprot.txt: "ID A0A2U1PS28_ARTAN Unreviewed; 661 AA. ... DR InterPro; IPR006297; EF-4. ... DR PANTHER; PTHR43512:SF4; TRANSLATION FACTOR GUF1 HOMOLOG, CHLOROPLASTIC; 1. ... FT DOMAIN 86..245 ... FT /note="Tr-type G""
PMID:23166764: "promotes the efficiency of chloroplast protein synthesis"