Inner membrane spanin subunit (i-spanin; PseT.3; gpY13K) is a 117-amino-acid single-pass type II membrane protein of bacteriophage T4 anchored in the Escherichia coli inner (plasma) membrane via an N-terminal transmembrane helix (residues 3-23) with a periplasmic coiled-coil domain (residues 24-117). Through C-terminal interaction with the outer-membrane lipoprotein o-spanin subunit Y13J (PseT.2), Y13K forms a periplasm-spanning two-component spanin complex that disrupts the host outer membraneโlikely by catalyzing inner-outer membrane fusion once the peptidoglycan layer has been degraded by endolysinโto complete host cell lysis and progeny virion release.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0020002 host cell plasma membrane | IEA GO_REF:0000044 | ACCEPT | Summary: Accurately reflects localization of the i-spanin subunit to the host bacterial inner (plasma) membrane. Reason: Y13K (PseT.3) possesses an N-terminal transmembrane helix (residues 3-23) that anchors it in the Escherichia coli inner (plasma) membrane as a type II integral membrane protein with its coiled-coil domain extending into the periplasm. Supporting Evidence: file:BPT4/y13K/y13K-deep-research-falcon.md **PseT.3 (y13K)** carries an **N-terminal transmembrane domain**, consistent with an **inner-membrane anchor** and a periplasm-facing domain. file:BPT4/y13K/y13K-uniprot.txt SUBCELLULAR LOCATION: Host cell inner membrane {ECO:0000250}; Single-pass type II membrane protein {ECO:0000250}; Periplasmic side {ECO:0000250}. |
| GO:0031640 killing of cells of another organism | IEA GO_REF:0000043 | MODIFY | Summary: Broad keyword-derived term that should be refined to the specific descendant GO:0044659 (viral release from host cell by cytolysis) already annotated to the gene. Reason: Y13K specifically mediates the terminal outer-membrane disruption step of phage-induced host cytolysis for virion exit, which is captured by the more specific descendant term GO:0044659 already present on this gene. Proposed replacements: viral release from host cell by cytolysis Supporting Evidence: PMID:17900620 In the case of T4, a deletion of a non-overlapping gene pair pseT.2 and pseT.3 identified as Rz/Rz1 equivalents resulted in the same divalent cation-dependent lysis phenotype. file:BPT4/y13K/y13K-deep-research-falcon.md **y13K/pseT.3 (UniProt P39504)** functions as the **i-spanin** subunit in the T4 spanin complex that is required for **outer-membrane disruption during the final step of host cell lysis**. |
| GO:0044659 viral release from host cell by cytolysis | IGI PMID:17900620 Rz/Rz1 lysis gene equivalents in phages of Gram-negative hos... | ACCEPT | Summary: Directly supported by T4 deletion genetics demonstrating that the pseT.3/pseT.2 spanin pair is required for host outer membrane disruption and cell lysis. Reason: Summer et al. (2007) demonstrated that deletion of pseT.3 (y13K) and pseT.2 (y13J) in bacteriophage T4 causes a divalent cation-dependent lysis defect in which spherical cells fail to complete outer membrane disruption and virion release. Supporting Evidence: PMID:17900620 In the case of T4, a deletion of a non-overlapping gene pair pseT.2 and pseT.3 identified as Rz/Rz1 equivalents resulted in the same divalent cation-dependent lysis phenotype. file:BPT4/y13K/y13K-deep-research-falcon.md A key defining experimental result is that deletion of the **non-overlapping gene pair pseT.3/pseT.2** in T4 produces the classical **divalent cation (Mg2+)-dependent lysis defect** known for ฮป Rz/Rz1 mutants: infected cells undergo PG removal but fail to complete OM disruption efficiently under Mg2+-supplemented conditions, forming **spherical** cells and yielding **small plaques**/plating defects. |
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Download this section (compressed HTML)Q: How does the periplasmic coiled-coil domain of T4 Y13K (residues 24-93) transition from a peptidoglycan-trapped extended rod to a post-fusion hairpin conformation upon peptidoglycan hydrolysis?
Q: Which specific C-terminal residues of Y13K (PseT.3) mediate high-affinity heterooligomerization with the outer-membrane lipoprotein subunit Y13J (PseT.2)?
Experiment: Perform circular dichroism spectroscopy and size-exclusion chromatography with multi-angle light scattering (SEC-MALS) on the purified periplasmic domain of Y13K (residues 24-117) alone and mixed with mature Y13J (residues 18-99) to characterize coiled-coil bundle formation upon complex assembly.
Experiment: Conduct spheroplast fusion assays in Escherichia coli co-expressing T4 Y13K and Y13J to directly demonstrate fusogenic activity of the T4 two-component spanin complex.
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