Holins and the phage infection cycle

Bacteriophage lambda Bacteriophage lysis is a precisely scheduled process controlled by proteins of the holin family. Holins are a diverse class of small phage-encoded membrane proteins. The best studied holin is S105, a 105-residue polypeptide with three transmembrane domains (TMDs) encoded by the S gene of phage λ. Throughout the period of late gene expression and particle assembly, S105 accumulates in the cytoplasmic membrane of Escherichia coli without any effect on its integrity. Suddenly, at a programmed time, S105 triggers to form a lesion, or hole, in the membrane; this allows the λ-endolysin, R, to escape from the cytoplasm and attack the cell wall. In phages of Gram-negative hosts, there is a third step to complete the lysis pathway involving a protein or protein complex, the spanin, which connects the cytoplasmic and outer membranes. In λ, the spanin complex consists of the cytoplasmic membrane protein, Rz, and the outer membrane lipoprotein, Rz1. This complex is thought to act by disrupting the outer membrane, possibly by fusion with the inner membrane. The large holes observed can be viewed as supporting the notion that at the time of lethal triggering, the S105 holin exists in such large aggregates, leading to one or a small number of holes rather than many smaller holes distributed throughout the membrane.

Micron-scale holes terminate the phage infection cycle. PNAS USA January 11 2010. doi: 10.1073/pnas.0914030107
Holins are small phage-encoded proteins that accumulate harmlessly in the cytoplasmic membrane during the phage infection cycle until suddenly, at an allele-specific time, triggering to form lethal lesions, or “holes.” In the phages λ and T4, the holes have been shown to be large enough to allow release of prefolded active endolysin from the cytoplasm, which results in destruction of the cell wall, followed by lysis within seconds. Here, the holes caused by S105, the λ-holin, have been captured in vivo by cryo-EM. Surprisingly, the scale of the holes is at least an order of magnitude greater than any previously described membrane channel, with an average diameter of 340 nm and some exceeding 1 μm. Most cells exhibit only one hole, randomly positioned in the membrane, irrespective of its size. Moreover, on coexpression of holin and endolysin, the degradation of the cell wall leads to spherically shaped cells and a collapsed inner membrane sac. To obtain a 3D view of the hole by cryo-electron tomography, we needed to reduce the average size of the cells significantly. By taking advantage of the coupling of bacterial cell size and growth rate, we achieved an 80% reduction in cell mass by shifting to succinate minimal medium for inductions of the S105 gene. Cryotomographic analysis of the holes revealed that they were irregular in shape and showed no evidence of membrane invagination. The unexpected scale of these holes has implications for models of holin function.

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