Perforin-1 (PRF) is a pore-forming protein encoded in humans by the PRF1 gene. It is stored in the secretory granules of cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells, collectively known as cytotoxic lymphocytes (CLs). Upon activation, these cells release perforin to form pores in the membranes of target cells, enabling the entry of granzymes that trigger apoptosis. Perforin is therefore a central effector molecule of the immune system, essential for the elimination of virus-infected and transformed cells. Mutations in PRF1 that impair perforin expression or function are associated with familial hemophagocytic lymphohistiocytosis (FHL) and related immune dysregulation syndromes, a spectrum of conditions sometimes collectively referred to as perforinopathies.
Structure
Perforin is a pore-forming cytolytic protein composed of approximately 555 amino acids and has a molecular weight of 60–70 kDa. The protein contains several domains: the conserved N-terminal membrane attack complex/perforin (MACPF) domain which is central to its pore-forming function, a C-terminal membrane-docking C2 domain responsible for calcium-dependent interaction with target membranes, and an epidermal growth factor (EGF)-like domain that provides flexibility and links the MACPF and C2 domains. The structure of perforin is further stabilized by nine disulfide bonds, and its N-terminal domain binds calcium ions, a key feature required for activation and subsequent insertion into lipid membranes. Oligomerization of approximately 20 perforin monomers forms large, cylindrical pores in target cell membranes; these pores are hydrophobic and disrupt ionic homeostasis, leading to cell death.
The lytic membrane-inserting region of perforin is the MACPF domain, which mediates pore formation. This domain shares homology with cholesterol-dependent cytolysins of Gram-positive bacteria. Perforin also shows structural similarity to complement component 9 (C9), another pore-forming protein that creates transmembrane tubules.
Purifying perforin has historically been difficult due to its loss of activity and stability in solution; only recently has a recombinant form been successfully produced.
Function
Perforin is a pore-forming cytolytic protein stored in the granules of cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells. Upon degranulation, perforin is escorted to the target cell membrane by calreticulin, a chaperone protein that prevents its premature degradation. Perforin binds to the target cell's plasma membrane through interactions with membrane phospholipids, while calcium ions enhance this binding by stabilizing interactions with phosphatidylcholine
Initially, perforin was thought to act only at the plasma membrane. However, subsequent findings revealed that granzyme B can be endocytosed independently of perforin. Washed cells that had internalized granzyme B underwent apoptosis when perforin was later added, even though perforin had not been present during endocytosis. These results led to the proposal that perforin's main function occurs at the endosomal rather than the plasma membrane, by disrupting endosomal integrity to release granzymes into the cytosol.
Through these mechanisms, perforin acts as a central effector molecule in CTL- and NK cell-mediated cytotoxicity.
Clinical significance
Familial hemophagocytic lymphohistiocytosis
Mutations in PRF1 that reduce or abolish perforin expression or pore-forming activity cause the autosomal-recessive disorder familial hemophagocytic lymphohistiocytosis (FHL) type 2 (FHL2). The loss of cytotoxic T lymphocyte (CTL) and natural killer (NK) cell function prevents effective granule-mediated cytotoxicity, leading to uncontrolled antigen presentation, T-cell hyperactivation, interferon-γ–driven macrophage activation, and severe hyperinflammation. PRF1 mutations account for roughly 20–50% of familial cases, with disease severity depending on mutation type: hypomorphic alleles with residual activity may present later in childhood or adulthood, whereas null mutations typically manifest in infancy.
Perforinopathy
The concept of "perforinopathy" encompasses a spectrum of disease presentations linked to impaired perforin function.
Interactions
Perforin has been shown to interact with calreticulin.
See also
- Granzymes
- Defensin
- Complement membrane attack complex