III

UniProt ID: P69168
Organism: Inovirus M13
Review Status: COMPLETE
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Gene Description

Minor coat and host attachment protein G3P (pIII), located in five copies complexed with five copies of pVI at the proximal (adsorption) tip of the filamentous M13 virion. Synthesized as a 424-residue precursor with an 18-residue signal peptide, mature pIII (residues 19-424) is anchored in the Escherichia coli inner membrane prior to virion assembly via a C-terminal transmembrane helix (residues 398-418) and comprises three functional domains (N1, N2, and CT) separated by flexible glycine-rich linkers. During infection, the N2 domain binds the tip of the host conjugative F-pilus (primary adhesion receptor); pilus retraction brings the virion to the cell envelope and unlocks the N1-N2 interface so that N1 binds the C-terminal domain of the inner-membrane co-receptor TolA, enabling TolQRA-dependent translocation of the viral genome into the host cytoplasm. At the conclusion of virion morphogenesis, the C-terminal domain of pIII integrates with pVI into the trailing tip of the nascent filament and mediates terminal release of the extruding virion from the host inner membrane.

Existing Annotations Review

GO Term Evidence Action Reason
GO:0019028 viral capsid
IEA
GO_REF:0000120 		MODIFY
Summary: Broad capsid localization refined to helical viral capsid (GO:0019029), where pIII forms a homopentamer at the proximal ("pointy" / head) tip of the mature M13 filamentous capsid.
Reason: M13 is a filamentous inovirus with a helical capsid architecture; refine the generic viral capsid term to helical viral capsid (GO:0019029), matching major coat protein pVIII and the other minor tip proteins.
Proposed replacements: helical viral capsid
Supporting Evidence:
file:BPM13/III/III-uniprot.txt
Prior to assembly, G3P is found associated with the bacterial host inner membrane. There are about five copies of this protein per mature phage that are located on the head side of the filamentous virion.
GO:0019062 virion attachment to host cell
IEA
GO_REF:0000043 		MODIFY
Summary: General attachment term superseded by the specific primary F-pilus attachment and secondary TolA entry receptor attachment terms already present on this gene.
Reason: pIII mediates two sequential, well-characterized attachment steps: binding to the host F-pilus tip via its N2 domain (GO:0039666) and subsequent binding to the inner-membrane co-receptor TolA via its N1 domain (GO:0098670).
Supporting Evidence:
file:BPM13/III/III-uniprot.txt
During the initial step of infection, G3P mediates adsorption of the phage to its primary receptor, the tip of host F-pilus. Attachment of the phage causes pilus retraction bringing the viral particle into close proximity of the host cell inner membrane.
file:BPM13/III/III-uniprot.txt
Binding to the host pilus initiates a change in the G3P conformation, allowing subsequent interaction with the host entry receptors TolA, TolQ and TolR and penetration of the viral DNA into the host cytoplasm.
GO:0033644 host cell membrane
IEA
GO_REF:0000044 		MODIFY
Summary: Broad host membrane localization that should be refined to host cell plasma membrane (GO:0020002), where pIII is anchored prior to virion incorporation.
Reason: Prior to assembly into the proximal virion tip, mature pIII is a single-pass type I transmembrane protein anchored specifically in the Escherichia coli inner (plasma) membrane.
Proposed replacements: host cell plasma membrane
Supporting Evidence:
file:BPM13/III/III-uniprot.txt
Prior to assembly, G3P is found associated with the bacterial host inner membrane. There are about five copies of this protein per mature phage that are located on the head side of the filamentous virion.
GO:0039666 virion attachment to host cell pilus
IEA
GO_REF:0000043 		ACCEPT
Summary: Core biological process of pIII, whose N2 domain binds the tip of the host conjugative F-pilus to initiate M13 infection.
Reason: The N2 domain of pIII directly recognizes and binds the tip of the host F-pilus as the primary receptor for phage adsorption.
Supporting Evidence:
file:BPM13/III/III-uniprot.txt
During the initial step of infection, G3P mediates adsorption of the phage to its primary receptor, the tip of host F-pilus. Attachment of the phage causes pilus retraction bringing the viral particle into close proximity of the host cell inner membrane.
GO:0046718 symbiont entry into host cell
IEA
GO_REF:0000043 		MODIFY
Summary: Broad entry parent term superseded by the specific pilus-attachment and entry-receptor attachment processes mediated by pIII.
Reason: The specific molecular contributions of pIII to host entry are primary adsorption to the F-pilus followed by pilus retraction (GO:0039666) and secondary binding to the TolA/TolQ/TolR entry receptor complex (GO:0098670).
Supporting Evidence:
file:BPM13/III/III-uniprot.txt
Binding to the host pilus initiates a change in the G3P conformation, allowing subsequent interaction with the host entry receptors TolA, TolQ and TolR and penetration of the viral DNA into the host cytoplasm.
GO:0098670 entry receptor-mediated virion attachment to host cell
IEA
GO_REF:0000043 		ACCEPT
Summary: Core biological process of pIII, whose N1 domain binds the C-terminal domain of the host inner-membrane co-receptor TolA after F-pilus binding.
Reason: Following F-pilus binding by the N2 domain, conformational unlocking of pIII exposes the N1 domain to bind the C-terminal domain of host TolA (crystallographically resolved in PDB: 1TOL), mediating entry receptor attachment.
Supporting Evidence:
file:BPM13/III/III-uniprot.txt
Binding to the host pilus initiates a change in the G3P conformation, allowing subsequent interaction with the host entry receptors TolA, TolQ and TolR and penetration of the viral DNA into the host cytoplasm.
GO:0098671 adhesion receptor-mediated virion attachment to host cell
IEA
GO_REF:0000043 		MODIFY
Summary: General adhesion-receptor attachment term superseded by the specific child term virion attachment to host cell pilus (GO:0039666).
Reason: The primary adhesion receptor for M13 is specifically the tip of the host conjugative F-pilus, which is captured by the more specific term virion attachment to host cell pilus (GO:0039666).
Supporting Evidence:
file:BPM13/III/III-uniprot.txt
During the initial step of infection, G3P mediates adsorption of the phage to its primary receptor, the tip of host F-pilus. Attachment of the phage causes pilus retraction bringing the viral particle into close proximity of the host cell inner membrane.
GO:0099045 viral extrusion
IEA
GO_REF:0000043 		ACCEPT
Summary: Core biological process of pIII, whose C-terminal domain caps the trailing end of the nascent filament with pVI and mediates release of the mature virion from the host membrane.
Reason: At the end of filamentous phage assembly, the C-terminal domain of pIII integrates with pVI into the proximal virion tip and catalyses terminal release of the membrane-anchored phage particle during extrusion.
Supporting Evidence:
file:BPM13/III/III-uniprot.txt
In the extrusion process, G3P mediates the release of the membrane-anchored virion from the cell via its C-terminal domain.
GO:0039660 structural constituent of virion
ISS
file:BPM13/III/III-uniprot.txt 		NEW
Summary: Five copies of pIII form the structural proximal ("pointy") tip complex of the mature M13 virion together with five copies of pVI.
Reason: Five subunits of pIII are an integral structural component of the proximal adsorption tip of the mature M13 virion, anchoring the receptor-binding N1-N2 domains to the capsid via its C-terminal domain and pVI.
Supporting Evidence:
file:BPM13/III/III-uniprot.txt
Prior to assembly, G3P is found associated with the bacterial host inner membrane. There are about five copies of this protein per mature phage that are located on the head side of the filamentous virion.

Core Functions

Proximal virion tip structural and host-attachment protein (homopentamer complexed with pVI) that mediates primary phage adsorption to the host F-pilus tip via its N2 domain, binds the secondary entry receptor TolA via its N1 domain, and terminates filamentous virion assembly and extrusion by releasing the nascent particle from the host inner membrane via its C-terminal domain.

Molecular Function:
structural constituent of virion
Directly Involved In:
virion attachment to host cell pilus entry receptor-mediated virion attachment to host cell viral extrusion
Cellular Locations:
host cell plasma membrane
Supporting Evidence:
  • file:BPM13/III/III-uniprot.txt
    During the initial step of infection, G3P mediates adsorption of the phage to its primary receptor, the tip of host F-pilus. Attachment of the phage causes pilus retraction bringing the viral particle into close proximity of the host cell inner membrane.
  • file:BPM13/III/III-uniprot.txt
    Binding to the host pilus initiates a change in the G3P conformation, allowing subsequent interaction with the host entry receptors TolA, TolQ and TolR and penetration of the viral DNA into the host cytoplasm. In the extrusion process, G3P mediates the release of the membrane-anchored virion from the cell via its C-terminal domain.
  • file:BPM13/III/III-uniprot.txt
    Prior to assembly, G3P is found associated with the bacterial host inner membrane. There are about five copies of this protein per mature phage that are located on the head side of the filamentous virion.

References

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Suggested Questions for Experts

Q: How do the transmembrane helices of pIII and pVI remodel the host inner membrane during TolQRA-assisted viral coat depolymerization and ssDNA translocation?

Q: What molecular signal triggers the lateral entry of the pIII-pVI complex into the pI/pXI assembly machine only after the full length of the viral (+)ssDNA has been coated by pVIII?

Suggested Experiments

Experiment: Determine the cryo-EM structure of the M13 proximal tip (pIII-pVI pentamer) bound to the tip of the Escherichia coli conjugative F-pilus.

Experiment: Reconstitute the pIII-TolQRA interaction in proteoliposomes to assay whether proton-motive force activation of TolQRA drives conformational rearrangement of the pIII C-terminal domain and uncoating of the pVIII capsid.

📚 Additional Documentation

Notes

(III-notes.md)

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