Holins: form and function in bacteriophage lysis
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Abstract: During the lytic cycle of most bacteriophages, a phageencoded peptidoglycandegrading activity is elaborated. At least four entirely distinct types of enzymes fulfill this role and are given the generic name endolysin. Endolysins characterized to date are synthesized without a signal sequence and thus accumulate fully folded and active in the cytosol during the vegetative phase. Small membrane proteins are required in order for endolysins to gain access to the peptidoglycan. Because the available data suggest that the membrane lesion formed by these proteins is stable and nonspecific, these proteins have been given the designation holins (holeformers). Analysis of the primary sequence suggests a simple membrane topology with two or more membranespanning helical domains and a highly charged, hydrophilic Cterminus. Comparison of the sequences of holins from phages of Gramnegative hosts suggests there are at least two major holin groups. Putative holin genes have also been found in bacteriophages of Grampositive bacteria. Altogether, in phages of Eubacteria, 11 or more unrelated gene families which share the functional and structural characteristics of holins have been identified. Genetic and physiological analysis suggest that holins are primarily regulated at the level of function. Holin function is modulated in some cases by a second protein encoded by the holin gene. The primary regulation of holin function, however, appears to be intrinsic to the holin structure itself, since a missense allele of the S holin gene of phage has been found which abolishes the normal delay that allows the vegetative phase to generate a useful number of progeny. Copyright 1995, Wiley Blackwell. All rights reserved
