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Bacteriophage lambda uses a holin-endolysin system for host cell lysis. R, the endolysin, has muralytic activity. S, the holin, is a small membrane protein that permeabilizes the inner membrane at a precisely scheduled time after infection and allows the endolysin access to its substrate, resulting in host cell lysis. lambda S has a single cysteine at position 51 that can be replaced by a serine without loss of the holin function. A collection of 27 single-cysteine products of alleles created from lambda S(C51S) were tested for holin function. Most of the single-cysteine variants retained the ability to support lysis. Mutations with the most defective phenotype clustered in the first two hydrophobic transmembrane domains. Several lines of evidence indicate that S forms an oligomeric structure in the inner membrane. Here we show that oligomerization does not depend on disulfide bridge formation, since the cysteineless S(C51S) (i) is functional as a holin and (ii) shows the same oligomerization pattern as the parental S protein. In contrast, the lysis-defective S(A52V) mutant dimerizes but does not form cross-linkable oligomers. Again, dimerization does not depend on the natural cysteine, since the cysteineless lysis-defective S(A52V/C51S) is found in dimers after treatment of the membrane with a cross-linking agent. Furthermore, under oxidative conditions, dimerization via the natural cysteine is very efficient for S(A52V). Both S(A52V) (dominant negative) and S(A48V) (antidominant) interact with the parental S protein, as judged by oxidative disulfide bridge formation. Thus, productive and unproductive heterodimer formation between the parental protein and the mutants S(A52V) and S(A48V), respectively, may account for the dominant and antidominant lysis phenotypes. Examination of oxidative dimer formation between S variants with single cysteines in the hydrophobic core of the second membrane-spanning domain revealed that positions 48 and 51 are on a dimer interface. These results are discussed in terms of a three-step model leading to S-dependent hole formation in the inner membrane.

Genetic and Biochemical Analysis of Dimer and Oligomer Interactions of the λ S Holin