Membrane assembly of the cholesterol-dependent cytolysin pore complex

Abstract

The cholesterol-dependent cytolysins (CDCs) are a large family of pore-forming toxins that are produced, secreted and contribute to the pathogenesis of many species of Gram-positive bacteria. The assembly of the CDC pore-forming complex has been under intense study for the past 20 years. These studies have revealed a molecular mechanism of pore formation that exhibits many novel features. The CDCs form large β-barrel pore complexes that are assembled from 35 to 40 soluble CDC monomers. Pore formation is dependent on the presence of membrane cholesterol, which functions as the receptor for most CDCs. Cholesterol binding initiates significant secondary and tertiary structural changes in the monomers, which lead to the assembly of a large membrane embedded β-barrel pore complex. This review will focus on the molecular mechanism of assembly of the CDC membrane pore complex and how these studies have led to insights into the mechanism of pore formation for other pore-forming proteins. This article is part of a Special Issue entitled: Protein Folding in Membranes.

Figure 1. Molecular structure of Clostridium perfringens perfringolysin O

Shown is a ribbon representation of the PFO crystal structure (PDB ID: 1PFO) [32]. Specific features of the structure are designated as D1-D4, domains 1-4; TMH1 and TMH2 (orange), transmembrane hairpins 1 and 2; β1, β4 and β5, β-strands 1, 4 and 5, α1, α-strand 1. L1-L3, loops L1-L3 The double glycine motif is shown as purple space filled atoms. All structures where generated using VMD [126]

Figure 2. The structure of the undecapeptide motif and location of the CRM

Shown in panel A is an overlay of domain 4 structures from Clostridium perfringens PFO (red), Streptococcus suis SLY (PDB ID: 3HVN) [35] (yellow), Bacillus anthracis ALO (PDB ID: 3CQF) [34] (cyan) and Streptococcus intermedius ILY (PDB ID: 1S3R) [33](blue). Shown in B is the location of the cholesterol recognition/binding motif of the same CDCs as in A (colors assignments are the same as in panel A).

Figure 3. The crystal structures of MACPF family proteins showing a CDC domain 3 like structure

The crystal structures of the MACPF proteins mouse perforin (PDB ID: 3NSJ) [88], complement C8 (PDB ID: 3OJY) [120] and the Photorhabdus luminescens Plu-MACPF (PDB ID: 2QP2) [89] are shown with that of PFO [32]. In each MACPF structure the PFO-like domain 3-like fold is highlighted. Shown in purple ribbon structure in each MACPF structure is the equivalent structure to the PFO α1-β5 loop that rotates away from β4. Shown in red spacefilled atoms is the conserved twin glycine motif of each protein that is present at the junction between β4 and β5, which is conserved in all know CDCs [46].