1.E.27.1.8
Plasmid-encoded UviB-like holin of 65 aas with 1 TMS, TpeE. This protein mediates non-lytic protein translocation (Brüser and Mehner-Breitfeld 2022). Holins generate large membrane lesions that permit the passage of endolysins across the cytoplasmic membrane of prokaryotes, ultimately resulting in cell wall degradation and cell lysis. However, there are  examples known for non-lytic holin-dependent secretion of proteins by bacteria, indicating that holins may transport proteins without causing large membrane lesions. Phage-derived holins can be used for a non-lytic endolysin translocation to permeabilize the cell wall for the passage of secreted proteins. In addition, clostridia, which do not possess the Tat pathway for transport of folded proteins, most likely employ non-lytic holin-mediated transport also for secretion of toxins and bacteriocins that are incompatible with the general Sec pathway. The small holin TpeE mediates non-lytic toxin secretion in Clostridium perfringens. TpeE contains only one short transmembrane helix that is followed by an amphipathic helix, which is reminiscent of TatA, the membrane-permeabilizing component of the Tat translocon for folded proteins. Brüser and Mehner-Breitfeld 2022 reviewed the known cases of non-lytic holin-mediated transport and then focus on the structural and functional comparison of TatA and TpeE, resulting in a mechanistic model for holin-mediated transport. This model is strongly supported by a so far not recognized naturally occurring holin-endolysin fusion protein.