Dimerization between the holin and holin inhibitor of phage lambda

Holins are integral membrane proteins that control the access of phage-enco ded muralytic enzymes, or endolysins, to the cell wall by the sudden format ion of an uncharacterized homo-oligomeric lesion, or hole, in the membrane, at a precisely defined time. The timing of lambda-infected cell lysis depe nds solely on the 107 codon S gene, which encodes two proteins, S105 and S1 07, which are the holin and holin inhibitor, respectively. Here we report t he results of biochemical and genetic studies on the interaction between th e holin and the holin inhibitor. A unique cysteine at position 51, in the m iddle of the second transmembrane domain, is shown to cause the formation o f disulfide-linked dimers during detergent membrane extraction. Forced oxid ation of membranes containing S molecules also results in the formation of covalently linked dimers. This technique is used to demonstrate efficient d imeric interactions between S105 and S107. These results, coupled with the previous finding that the timing of lysis depends on the excess of the amou nt of S105 over S107, suggest a model in which the inhibitor functions by t itrating out the effector in a stoichiometric fashion. This provides a basi s for understanding two evolutionary advantages provided by the inhibitor s ystem, in which the production of the inhibitor not only causes a delay in the timing of lysis, allowing the assembly of more virions, but also increa ses effective hole formation after triggering.