Curation Notes: Bacteriophage T4 y13J (UniProt P39503)

Identity and Nomenclature

• Gene symbol: y13J (synonym: pseT.2)
• UniProt accession: P39503 (SPAN2_BPT4)
• Protein name: Spanin, outer lipoprotein subunit (o-spanin)
• Length: 99 AA precursor (signal peptide 1-16; mature lipoprotein chain 17-99, lipidated at Cys17)
• Organism: Enterobacteria phage T4 (NCBITaxon:10665)

Summary of Experimental Evidence

• Identification and Genetic Validation in T4 (Summer et al. 2007): [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."] y13J (pseT.2) encodes the outer-membrane lipoprotein subunit (o-spanin) of the T4 two-component spanin complex, paired in a separated head-to-tail gene architecture with y13K (pseT.3), which encodes the inner-membrane subunit (i-spanin). Deletion of pseT.2/pseT.3 in T4 causes a divalent cation ($Mg^{2+}$)-dependent lysis defect in which infected E. coli cells become spherical after holin and endolysin action because the outer membrane fails to disrupt.
• Subcellular Localization and Mechanism: Synthesized as a 99-AA precursor with an N-terminal signal peptidase II lipobox (SIGNAL 1..16), y13J (pseT.2) is lipidated on Cys17 (N-palmitoyl cysteine, S-diacylglycerol cysteine) and sorted to the periplasmic leaflet of the host cell outer membrane (SUBCELLULAR LOCATION: Host cell outer membrane {ECO:0000250}; Lipid-anchor {ECO:0000250}; Periplasmic side {ECO:0000250}.). Its C-terminal periplasmic domain interacts with the C-terminus of the i-spanin y13K (pseT.3) to form a periplasm-spanning complex (These proteins were named "spanins," since their protein products are predicted to span the periplasm providing a physical connection between the inner and outer membranes. PMID:17900620). After endolysin degrades the peptidoglycan meshwork, the spanin complex drives fusion of the inner and outer membranes to disrupt the outer membrane barrier and complete host cell lysis.

Review of Existing GO Annotations

1. GO:0044659 (viral release from host cell by cytolysis, IGI PMID:17900620): ACCEPT. Directly supported by the $\Delta(\text{pseT.2 pseT.3})$ lysis defect in T4 PMID:17900620.
2. GO:0031640 (killing of cells of another organism, IEA GO_REF:0000043): MODIFY to GO:0044659 (viral release from host cell by cytolysis), which is the specific descendant process already experimentally annotated on the gene.
3. GO:0020002 (host cell plasma membrane, IEA GO_REF:0000044): MODIFY to GO:0033644 (host cell membrane). In Gram-negative bacteria (E. coli), the plasma membrane is the inner membrane (where the partner i-spanin y13K resides), whereas y13J (pseT.2) is an outer-membrane lipoprotein anchored in the Host cell outer membrane (UniProtKB-SubCell:SL-0376). Because GO_REF:0000044 conservatively mapped SL-0376 to GO:0020002, replacing GO:0020002 with GO:0033644 (host cell membrane) avoids misplacing the outer-membrane lipoprotein subunit in the inner (plasma) membrane.