MACPF

The MACPF protein superfamily is named after a domain that is common to the Membrane Attack Complex proteins (MAC) of complement (C6, C7, C8α, C8β and C9) and perforin (PF). Many members of this protein family are important eukaryote pore forming toxins.

The archetypal members of the family are complement C9 and perforin, both of which function in human immunity. C9 functions by punching holes in the membrane of pathogenic Gram-negative bacteria. Perforin is released by cytotoxic T cells and lyses virally infected and transformed cells. In addition perforin permits delivery of cytotoxic proteases called granyzmes that cause cell death. Deficiency of either protein can result in human disease. Structural studies reveal that MACPF domains are related to cholesterol dependent cytolysins (CDCs), a family of pore forming toxins previously thought to only exist in bacteria.

Biological roles of MACPF domain containing proteins
To date, around 500 members of the MACPF superfamily have been identified. Many of these proteins play key roles in the plant and animal immunity.

The complement proteins C6-C9 all contain a MACPF domain and assemble into the membrane attack complex. C6, C7 and C8β appear to be non-lytic and function as scaffold proteins within the MAC. In contrast both C8α and C9 are capable of lysing cells. The final stage of MAC formation involves polymerisation of C9 into a large pore that punches a hole in the outer membrane of Gram negative bacteria.

Perforin is stored in granules within cytotoxic T-cells and is responsible for killing virally infected and transformed cells. Perforin functions via two distinct mechanisms. Firstly, like C9, high concentrations of perforin can form pores that lyse cells. Secondly, perforin permits delivery of the cytotoxic granyzmes A and B into target cells. Once delivered, granyzmes are able to induce apoptosis and cause target cell death.

The plant protein CAD1 functions in the plant immune response to bacterial infection.

The sea anemone Actineria villosa uses a MACPF protein as a lethal toxin. MACPF proteins are also important for the invasion of the Malarial parasite into the mosquito host and the liver.

Not all MACPF proteins function in defence or attack. For example, astrotactin is involved in neural cell migration in mammals and apextrin is involved in sea urchin (Heliocidaris erythrogramma) develpoment. Drosophila Torso-like protein, which controls embryonic patterning, also contains a MACPF domain. It is unknown whether the function of any of these proteins involves lytic activity.

Functionaly uncharacterised MACPF proteins are sporadically distributed in bacteria. Several species of Chlamydia contain MACPF proteins. The insect pathogenic bacteria Photorhabdus luminescens also contains a MACPF protein, however, this molecule appears non-lytic.

Structure and mechanism
The X-ray crystal structure of Plu-MACPF, a protein from the insect pathogenic enterobacteria Photorhabdus luminescens has been determined (figure 1). These data reveal that the MACPF domain is homologous to pore forming cholesterol dependent cytolysins (CDC's) from Gram positive pathogenic bacteria such as Clostridium perfringens (which causes gas gangrene). The amino acid sequence identity between the two families is extremely low, and the relationship is not detectable using conventional sequnce based data mining techniques.

It is suggested that MACPF proteins and CDCs form pores in the same way (figure 1). Specifically it is hypothesised that MACPF proteins oligomerise to form a large circular pore (figure 2). A concerted conformational change within each monomer then results in two α-helical regions unwinding to form four amphipathic β-strands that span the membrane of the target cell. Like CDC's MACPF proteins are thus β-pore forming toxins that act like a molecular hole punch.

Control of MACPF proteins
Complement regulatory proteins such as CD59 function as MAC inhibitors and prevent inappropriate activity of complement against self cells (Figure 3). Biochemical studies have revealed the peptide sequences in C8α and C9 that bind to CD59. Analysis of the MACPF domain structures reveals that these sequences map to the second cluster of helices that unfurl to span the membrane. It is therefore suggested that CD59 directly inhibits the MAC by interfering with conformational change in one of the membrane spanning regions.

Other proteins that bind to the MAC include C8γ. This protein belongs to the lipocalin family and interacts with C8α. The binding site on C8α is known, however, the precise role of C8γ in the MAC remains to be understood.

Role in Human disease
Deficiency of C9, or other components of the MAC results in an increased susceptibity to diseases caused by Gram negative bacteria such as meningococcal meningitis. Overactivity of MACPF proteins can also cause disease. Most notably, deficiency of the MAC inhibitor CD59 results in an overactivity of complement and Paroxysmal nocturnal hemoglobinuria.

Perforin deficiency results in the commonly fatal disorder familial hemophagocytic lymphohistiocytosis (FHL or HLH). This disease is characterised by an overactivation of lymphocytes which results in cytokine mediated organ damage.

The MACPF protein DBCCR1 may function as a tumour suppressor in bladder cancer.