Major capsid protein gp23 (521 amino acids; cleaved after Glu65 by the prohead protease gp21 to yield the 456-residue mature gp23*) is the principal structural building block of the bacteriophage T4 head. Folded in the Escherichia coli cytoplasm with the obligatory assistance of the host GroEL chaperonin and phage co-chaperonin gp31, gp23 polymerizes into 155 hexameric capsomers (930 monomers) around the prohead scaffolding core to form the hexagonal surface lattice of the prolate icosahedral capsid (Tend = 13 laevo, Tmid = 20), which subsequently undergoes proteolytic cleavage, conformational expansion during DNA packaging, and external clamping by the decoration proteins Soc and Hoc.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0019028 viral capsid | IEA GO_REF:0000120 | MODIFY | Summary: gp23* forms the hexameric capsomer lattice of the T4 capsid, which is more specifically described by the descendant term GO:0039621 (T=13 icosahedral viral capsid). Reason: GO:0019028 (viral capsid) is accurate but less specific than GO:0039621 (T=13 icosahedral viral capsid), which is already present on this record. Proposed replacements: T=13 icosahedral viral capsid Supporting Evidence: file:BPT4/gp23/gp23-uniprot.txt The major capsid protein self-associates to form 160 hexamers, building most of the T=13 laevo capsid. file:BPT4/gp23/gp23-deep-research-falcon.md In the mature virion it is present as the **proteolytically processed** form **gp23***, which assembles into **hexameric capsomers** to build the **hexagonal lattice** of the **prolate icosahedral** head. |
| GO:0039621 T=13 icosahedral viral capsid | IEA GO_REF:0000043 | ACCEPT | Summary: gp23* hexamers form the prolate icosahedral capsid lattice with Tend = 13 laevo icosahedral end caps (and T = 13 isometric capsids in point mutants). Reason: Cryo-EM reconstructions establish that 155 hexamers of mature gp23* form the T4 capsid lattice characterized by T = 13 laevo icosahedral end caps (and Tmid = 20 in the prolate cylinder). Supporting Evidence: file:BPT4/gp23/gp23-uniprot.txt The major capsid protein self-associates to form 160 hexamers, building most of the T=13 laevo capsid. file:BPT4/gp23/gp23-deep-research-falcon.md In the mature virion it is present as the **proteolytically processed** form **gp23***, which assembles into **hexameric capsomers** to build the **hexagonal lattice** of the **prolate icosahedral** head. |
| GO:0019028 viral capsid | IDA PMID:4920094 Analysis of T4 phage proteins. I. Conversion of precursor pr... | MODIFY | Summary: Hosoda & Cone (1970, PMID:4920094) demonstrated that P23 (gp23) is cleaved into the major capsid polypeptide (A-protein / gp23*) accounting for ~60% of total capsid protein; refine to GO:0039621 for consistency with the more specific capsid term on this gene. Reason: Refined to the more specific descendant term GO:0039621 (T=13 icosahedral viral capsid) already present on this record. Proposed replacements: T=13 icosahedral viral capsid Supporting Evidence: PMID:4920094 conversion of P23 into A-protein and alteration of the protein coded by gene 22 (P22) appeared to be vital steps in formation of normal capsids. |
| GO:0039660 structural constituent of virion | NAS file:BPT4/gp23/gp23-deep-research-falcon.md | NEW | Summary: gp23* is the principal structural protein of the T4 capsid, present in 930 copies (155 hexamers) per mature virion head. Reason: Mature gp23* constitutes the bulk of the virion head shell (~60% of total capsid protein mass). Supporting Evidence: file:BPT4/gp23/gp23-uniprot.txt The major capsid protein self-associates to form 160 hexamers, building most of the T=13 laevo capsid. file:BPT4/gp23/gp23-deep-research-falcon.md In the mature virion it is present as the **proteolytically processed** form **gp23***, which assembles into **hexameric capsomers** to build the **hexagonal lattice** of the **prolate icosahedral** head. |
| GO:0019069 viral capsid assembly | NAS PMID:4920094 Analysis of T4 phage proteins. I. Conversion of precursor pr... | NEW | Summary: Polymerization of gp23 around the scaffolding core followed by gp21-mediated removal of its 65-residue N-terminal delta-peptide and lattice expansion are essential steps in T4 capsid morphogenesis. Reason: Both precursor assembly into the prohead shell and proteolytic conversion of gp23 to gp23* are required for normal capsid formation. Supporting Evidence: PMID:4920094 conversion of P23 into A-protein and alteration of the protein coded by gene 22 (P22) appeared to be vital steps in formation of normal capsids. |
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Download this section (compressed HTML)Q: How does the N-terminal 65-residue delta-peptide of precursor gp23 interact with the gp22 scaffolding core to specify the prolate elongation (Tmid = 20) versus isometric (T = 13) geometry prior to gp21 cleavage?
Q: What structural features of unfolded gp23 prevent its productive folding inside the standard E. coli GroEL-GroES Anfinsen cage and necessitate replacement of GroES by the phage co-chaperonin gp31?
Experiment: Solve the high-resolution cryo-EM structure of the unexpanded, uncleaved prolate prohead (in a gp21-deficient mutant) to visualize the N-terminal delta-domain of gp23 in complex with the internal gp22 scaffolding core.
Experiment: Determine the cryo-EM structure of non-native gp23 encapsulated within the GroEL-gp31 chaperonin chamber to identify the steric and electrostatic contacts that require the expanded gp31 folding cavity.
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