Gene Ontology annotation based on UniProtKB/Swiss-Prot keyword mapping

The C-terminal fragment of the precursor tail lysozyme of bacteriophage T4 stays as a structural component of the baseplate after cleavage.

The tail-associated lysozyme of phage T4 is the product of gene 5, an essential structural component of the baseplate hub, synthesised as a 63 kDa precursor that is cleaved to a 43 kDa mature protein.

"Tail-associated lysozyme of bacteriophage T4 (tail lysozyme), the product of gene 5 (gp 5), is an essential structural component of the hub of the phage baseplate. It is synthesized as a 63-kDa precursor, which later cleaves to form mature gp 5 with a molecular weight of 43,000."
The C-terminal cleavage fragment of the gp5 precursor (gp5C) remains in the tail and in the phage particle, so it is a structural component of the baseplate.

"Surprisingly, the C-terminal fragment was found in the tail and the phage particle by immunoblotting."

PMID:23925114

PAAR-repeat proteins sharpen and diversify the type VI secretion system spike.

gp5.4 is the bacteriophage T4 member of the PAAR-repeat superfamily, identified as the small protein that binds the tip of the gp5 beta-helix in the baseplate.

"Proteins containing the PAAR-repeat motif were strongly represented in this group with gp5.4 being the corresponding protein of T4 phage."
A PAAR protein folds into a cone with a sharp tip and a triangular base that fully occupies the blunt end of the spike beta-helix.

"In both complexes, a single chain of the PAAR protein folds into a symmetrical cone-shaped structure with a sharp tip and a triangular base fully occupying the blunt end of the β-helix"
The PAAR fold is stabilised by a zinc ion bound by three histidines and one cysteine, a site conserved across close homologues, which holds the pointed tip together during envelope penetration.

"The fold of the PAAR protein is stabilized by a Zn atom positioned close to the cone’s vertex"
Because the PAAR protein caps the spike, it is the piercing tip that makes the initial opening in the target cell envelope.

"Because a PAAR-repeat protein caps the end of the β-helix of a VgrG spike, it is in fact the piercing tip that is responsible for the initial event of creating an opening in the target cell envelope."

PMID:27193680

Structure of the T4 baseplate and its function in triggering sheath contraction.

The atomic structure of the 6 MDa T4 baseplate was determined in pre- and post-host-attachment states, identifying the components of the assembly and the mechanism that triggers sheath contraction.

"Here we present the atomic structure of the approximately 6-megadalton bacteriophage T4 baseplate in its pre- and post-host attachment states and explain the events that lead to sheath contraction in atomic detail."

PMID:3157805

Isolation and characterization of the bacteriophage T4 tail-associated lysozyme.

The T4 tail-associated lysozyme was purified from tails and identified unambiguously as gp5, a 42 kDa component of the baseplate hub, with N-acetylmuramidase activity matching the gene e lysozyme.

"Direct evidence has been obtained that the tail-associated lysozyme of bacteriophage T4 (tail-lysozyme) is gp5, which is a protein component of the hub of the baseplate."
The purified tail lysozyme has N-acetylmuramidase activity with the same substrate specificity as the gene e lysozyme but a lower pH optimum.

"The tail-lysozyme has N-acetylmuramidase activity and the same substrate specificity as gene e lysozyme; the optimum pH is around 5.8, about 1 pH unit lower than for the e lysozyme."