Holins are a diverse group of small proteins produced by dsDNA bacteriophages in order to trigger and control the degradation of the host's cell wall at the end of the lytic cycle.
[1] Over 50 unrelated gene families encode holins, making them the most diverse group of proteins with common function.
They are associated with SAR endolysins, which remain inactive in the periplasm prior to the depolarization of the membrane.
[5] Viruses that infect eukaryotic cells may use similar channel-forming proteins called viroporins.
[3] Class I holins have three transmembrane domains (TMDs) with the N-terminus in the periplasm and the C-terminus in the cytoplasm.