Schematic representations of the lysis genes and proteins of C. crescentus phages. Lysis components of phages phiCbK and Colossus are compared to those of other phages. A: Overall organization of the lysis cassettes of phages lambda, P2, phiCbK and Colossus; genes are represented by colored boxes, with gene functions labeled above each module and gene names below. Color indicates conserved function, not sequence similarity. B: Protein sequence alignment of the N-termini of the phages P1, 21, phiCbK and Colossus endolysins; the positions of equivalent E-D/C-T catalytic residues are highlighted in red, green and blue, transmembrane SAR domains are highlighted in grey. Identical residues in phiCbK gp104 and P1 Lyz are indicated by vertical bars, illustrating their relationship. C: The mature spanin proteins of phages lambda, P2 and phiCbK. Transmembrane domains are purple, predicted alpha-helical domains are blue, unstructured domains are black curves, and the positions of proline residues are represented by red dots. The spanin IM components are anchored to the inner membrane by a transmembrane helix, OM components are tethered to the outer membrane by a lipoylated (red bars) Cys residue (green dot). The P2 complex contains alpha-helical IM and OM components with proline-rich domains, and the phiCbK spanin complex more closely resembles that of P2. D: Possible membrane topologies of the phiCbK-like holin proteins. Left, three TMDs with N-in, C-out topology; right, two TMDs — including a 36-residue TMD2 — with N-in, C-in topology. Charge distributions of the protein and TMD length strongly favor the three TMD model.
Gill et al. BMC Genomics 2012 13:542 doi:10.1186/1471-2164-13-542