Phage lysis: Three steps, three choices, one outcome

Abstract

The lysis of bacterial hosts by double-strand DNA bacteriophages, once thought to reflect merely the accumulation of sufficient lysozyme activity during the infection cycle, has been revealed to recently been revealed to be a carefully regulated and temporally scheduled process. For phages of Gram-negative hosts, there are three steps, corresponding to subversion of each of the three layers of the cell envelope: inner membrane, peptidoglycan, and outer membrane. The pathway is controlled at the level of the cytoplasmic membrane. In canonical lysis, a phage encoded protein, the holin, accumulates harmlessly in the cytoplasmic membrane until triggering at an allele-specific time to form micron-scale holes. This allows the soluble endolysin to escape from the cytoplasm to degrade the peptidoglycan. Recently a parallel pathway has been elucidated in which a different type of holin, the pinholin, which, instead of triggering to form large holes, instead triggers to form small, heptameric channels that serve to depolarize the membrane. Pinholins are associated with SAR endolysins, which accumulate in the periplasm as inactive, membrane-tethered enzymes. Pinholin triggering collapses the proton motive force, allowing the SAR endolysins to refold to an active form and attack the peptidoglycan. Surprisingly, a third step, the disruption of the outer membrane is also required. This is usually achieved by a spanin complex, consisting of a small outer membrane lipoprotein and an integral cytoplasmic membrane protein, designated as o-spanins and i-spanins, respectively. Without spanin function, lysis is blocked and progeny virions are trapped in dead spherical cells, suggesting that the outer membrane has considerable tensile strength. In addition to two-component spanins, there are some single-component spanins, or u-spanins, that have an N-terminal outer-membrane lipoprotein signal and a C-terminal transmembrane domain. A possible mechanism for spanin function to disrupt the outer membrane is to catalyze fusion of the inner and outer membranes.

Antimicrobial bacteriophage-derived proteins and therapeutic applications

“…1A). 16 This well-choreographed event occurs after the endolysin has accumulated in the cytoplasm, and translocated through holes formed in the plasma membrane by holins. Since the peptidoglycan layer provides structural integrity and rigidity to the cell, degrading this layer leads to cell wall instability and eventual rupture due to differences in cellular and environmental osmotic pressures (osmolysis) (Figs.…”

“…1A). 15,16 As mentioned, endolysins are responsible for peptidoglycan degradation at the final stages of cell lysis. Endolysins are aided by two fundamentally different pathways involving holins, namely holin-endolysin and pinholin-SAR (signal anchor-release) endolysin systems.…”

“…For phage l, interruptions in the cytoplasmic membrane by holins occur from the formation of 'holes' ranging in size up to > 1mm. 16 The endolysins in the pinhole-SAR endolysin system are exported by the host sec system into the periplasm, and accumulate as inactive proteins tethered to the membrane by their N-terminal SAR domain. Deenergization of the cytoplasmic membrane by imbedded pinholes activates the SAR endolysins.…”

“…Holins are responsible for disruption of the cytoplasmic membrane to assist endolysins in cell lysis. The origin and remarkable diversity of endolysins, 10,11 VAPGH, 13 depolymerases 14 and holins 15,16 have been reviewed previously.…”

Molecular microbiology in antibacterial research

“…The premise that phages have intelligently evolved multiple strategies to interfere with bacterial physiology enables us to study and exploit them and their gene products for new-concept antibacterial research. In this regard, two reviews are presented that cover phages and phage products: the one by BondyDenomy and Davidson (2014) focuses on lysogeny-mediated changes in bacterial physiology, whereas the other by Young (2014) describes the molecular mechanisms of bacterial cell lysis caused by phage proteins. Once a temperate phage lysogenizes its bacterial host cell, it can affect the physiological traits of the bacterial lysogen, which can be regarded as a symbiotic state: superinfection exclusion, virulence, motility, antibiotic resistance and other traits can be affected by the balance between prophage activity and bacterial immunity, in part represented by the CRISPR (clustered regularly interspaced short palindromic repeats)-mediated adaptive immune system.…”