The Actinobacterial Phage Holin (APH) Family (TC# 1.E.57) is a fairly large family of proteins between 105 and 180 amino acyl residues in length, typically exhibiting a single transmembrane segment (TMS) near the N-terminus.
A representative list of proteins belonging to the APH family can be found in the Transporter Classification Database.
A second putative holin gene (gp5) encoding a protein (Gp5) with a predicted single N-terminal TMS was identified at the end of the Ms6 lytic operon.
Neither the putative class II holin nor the single TMS polypeptide could trigger lysis in pairwise combinations with the endolysin LysA in Escherichia coli.
This suggests that in Ms6 infection, the correct and programmed timing of lysis is achieved by the combined action of Gp4 and Gp5.