Holins in bacteria, eukaryotes, and archaea: multifunctional xenologues with potential biotechnological and biomedical applications

Abstract

Holins form pores in the cytoplasmic membranes of bacteria for the primary purpose of releasing endolysins that hydrolyze the cell wall and induce cell death. Holins are encoded within bacteriophage genomes, where they promote cell lysis for virion release, and within bacterial genomes, where they serve a diversity of potential or established functions. These include (i) release of gene transfer agents, (ii) facilitation of programs of differentiation such as those that allow sporulation and spore germination, (iii) contribution to biofilm formation, (iv) promotion of responses to stress conditions, and (v) release of toxins and other proteins. There are currently 58 recognized families of holins and putative holins with members exhibiting between 1 and 4 transmembrane α-helical spanners, but many more families have yet to be discovered. Programmed cell death in animals involves holin-like proteins such as Bax and Bak that may have evolved from bacterial holins. Holin homologues have also been identified in archaea, suggesting that these proteins are ubiquitous throughout the three domains of life. Phage-mediated cell lysis of dual-membrane Gram-negative bacteria also depends on outer membrane-disrupting "spanins" that function independently of, but in conjunction with, holins and endolysins. In this minireview, we provide an overview of their modes of action and the first comprehensive summary of the many currently recognized and postulated functions and uses of these cell lysis systems. It is anticipated that future studies will result in the elucidation of many more such functions and the development of additional applications.

FIG 1

Schematic view of the topologies of members of the seven holin superfamilies (I to VII) (Table 1). Solid lines represent TMSs that are present in all known members of the superfamily. Dashed lines represent TMSs present in some but not all members. # TMSs, the number of transmembrane α-helical segments (TMSs) predicted for the proteins of a superfamily. Family numbers refer to the families in subclass 1.E of the Transporter Classification Database (TCDB; www.tcdb.org). (Reprinted from the work of Reddy and Saier in Biochimica et Biophysica Acta [5].)

FIG 2

Schematic depiction of the proteins involved in Gram-negative bacterial cell envelope disruption by holin-type lysis systems. Pinholins form small heptameric pores that collapse the membrane potential (the PMF) across the inner membrane, while the more conventional holins form large multisubunit pores of variable sizes that allow release of fully folded endolysins from the cytoplasm, which in the periplasm hydrolyze specific bonds in the peptidoglycan cell wall, depending on the lysin type. One- or two-component spanins disrupt the outer membrane by an unknown mechanism, possibly involving fusion of the outer membrane with the inner membrane. C⁺, a cation.