Figure 6.

Schematic comparison between current models of PRF and MPEG1 assembly. (A) At the T-cell immune synapse, vesicles (i) containing PRF [only the MAPCF (blue) and C2 domains (green) are shown] and granzymes (purple) fuse with the plasma membrane releasing their contents onto the target cell (ii). Within these cytotoxic granules (i), PRF is kept in a Ca²⁺-deficient environment at low pH, therefore PRF is unable to bind membranes. Upon being released into the immune synapse (ii), PRF encounters Ca²⁺ (zoomed inset; red spheres) and therefore, binds the lipid bilayer via a target recognition C2 domain (iii). PRF begins to oligomerize into arcs (iv) and, later, fully formed pores (vi). In the PRF mechanism, early arc intermediates can puncture the lipid membrane (v); these can continue to grow in a continuous manner by recruiting monomers or other arcs. Functional arcs that have punctured into the lipid bilayer are depicted with a white membrane lesion (v). The final PRF pore enables granzyme B (GrB; purple) to diffuse into the target cell (vii). (B) EEA1⁺ vesicles containing MPEG1 (i) are triggered to traffic toward and fuse with the phagosomal membrane by monoubiquitination (blue diamond) (ii). Tethered MPEG1 [only the MACPF (red) and MABP domains (yellow) are shown] is proteolyzed from the lipid bilayer [transmembrane region and cytosolic tail are shown as a line (green)] (iii). Cleaved MPEG1 oligomerizes into a prepore (v). Upon strong acidification (pH < 5), MPEG1 is activated and transitions into a pore (vi or ix). MPEG1 may follow two proposed pathways (iv or vii). In the trans-pore model, oligomerization occurs on the host bilayer (iv to v) and trans-pores breach the bilayer of target membranes in close proximity (vi) (82). Other receptor complexes may be required to drive the formation of a close membrane–membrane junction (blue/orange receptor complex; asterisk). Alternatively, MPEG1 monomers diffuse within the synapse (vii) and oligomerize on microbial bilayers (vii to viii). The MACPF or MABP domains rotate, to re-orient the MACPF machinery toward the microbial bilayer [vii or viii; unclear (83)]. A cis-pore breaches the microbial bilayer (ix). The stage of MACPF or MABP domain rotation is unclear. After either a trans- or cis-pore has formed, effector molecules enter the target cell via the MPEG1 pore (x).