gp38 (183 amino acids, ~22.3 kDa) is an essential, nonstructural gene-specific molecular chaperone (tail fiber assembly protein) of bacteriophage T4 required for the folding and homotrimerization of the distal long tail fiber adhesin subunit gp37 in the infected Escherichia coli cytoplasm. Acting in concert with the general phage tail-fiber chaperone gp57A, gp38 transiently recognizes and binds the C-terminal receptor-binding tip region of nascent gp37 monomers, preventing off-pathway aggregation and nucleating the C-to-N-terminal zippering and iron-coordinated beta-helix folding of the gp37 homotrimer before dissociating prior to virion incorporation.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0098003 viral tail assembly | IEA GO_REF:0000043 | MODIFY | Summary: gp38 specifically catalyzes the folding and assembly of the distal long tail fiber subunit gp37, so the broader parent term viral tail assembly should be refined to GO:0098004 (virus tail fiber assembly). Reason: gp38 is dedicated exclusively to the folding and trimerization of the long tail fiber adhesin gp37 and has no role in tail tube, sheath, or baseplate hub assembly. Because the more specific descendant term GO:0098004 (virus tail fiber assembly) is already present on this record, GO:0098003 is modified to GO:0098004. Proposed replacements: virus tail fiber assembly Supporting Evidence: file:BPT4/38/38-uniprot.txt Chaperone involved, together with gp57, in the assembly of |
| GO:0098004 virus tail fiber assembly | IEA GO_REF:0000043 | ACCEPT | Summary: gp38 is an essential phage-encoded chaperone required for the folding and homotrimerization of the distal long tail fiber protein gp37. Reason: Genetic, biochemical, and heterologous co-expression studies establish that in the absence of gp38, gp37 polypeptides fail to trimerize and accumulate as insoluble aggregates, completely blocking distal long tail fiber assembly. GO:0098004 accurately captures its core biological process. Supporting Evidence: file:BPT4/38/38-uniprot.txt Chaperone involved, together with gp57, in the assembly of file:BPT4/38/38-deep-research-falcon.md Multiple independent sources converge that **T4 gp38 is required for correct folding and trimeric assembly of gp37**, the distal LTF protein. |
| GO:0044183 protein folding chaperone | IEA file:BPT4/38/38-uniprot.txt | NEW | Summary: gp38 functions as a dedicated molecular chaperone that catalyzes the folding and trimerization of gp37 without being incorporated into the mature virion. Reason: Existing GOA annotations for gp38 lack a molecular function term. Because gp38 transiently binds the C-terminal region of unfolded gp37 monomers to prevent aggregation and promote productive homotrimerization before dissociating, GO:0044183 (protein folding chaperone) accurately represents its core molecular function. Supporting Evidence: file:BPT4/38/38-uniprot.txt Chaperone involved, together with gp57, in the assembly of file:BPT4/38/38-deep-research-falcon.md Multiple independent sources converge that **T4 gp38 is required for correct folding and trimeric assembly of gp37**, the distal LTF protein. |
| GO:0030430 host cell cytoplasm | IEA file:BPT4/38/38-deep-research-falcon.md | NEW | Summary: gp38 is a nonstructural phage protein that acts in the host bacterial cytoplasm during late infection and is absent from mature T4 virions. Reason: Unlike the gp38 orthologs of phages T2 and Mu that remain bound to the fiber tip, T4 gp38 dissociates after catalyzing gp37 folding and functions exclusively in the infected host cell cytoplasm (GO:0030430). Supporting Evidence: file:BPT4/38/38-deep-research-falcon.md The dominant model for T4 is that gp38 acts during **intracellular assembly/morphogenesis** of LTF components and is **not incorporated into the mature T4 long tail fiber** (nor detected in mature virions), consistent with its role as a chaperone rather than a structural receptor-binding adhesin in T4. |
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Download this section (compressed HTML)Q: What is the three-dimensional structure of the transient complex between T4 gp38 and the unfolded C-terminal domain of gp37, and what triggers gp38 release once the Fe2+-coordinated beta-helix of the gp37 trimer is formed?
Q: How do gp38 and the general tail-fiber chaperone gp57A coordinate their interactions along nascent gp37 polypeptide chains during co-translational or post-translational trimer assembly?
Experiment: Trap a pre-folding complex between T4 gp38 and a C-terminal truncation or His-box mutant of gp37 (unable to bind Fe2+ and complete folding) for structural determination by cryo-EM or X-ray crystallography.
Experiment: Perform in vitro refolding and fluorescence cross-correlation spectroscopy (FCCS) assays with purified gp37, gp38, gp57A, and Fe2+ to measure the stoichiometry, order of binding, and dissociation kinetics of gp38 during gp37 trimerization.
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