能夠形成微孔的免疫蛋白“perforin”是消除被病毒感染的細胞及癌變細胞所必需的，由自然殺手及細胞毒性T-細胞釋放。
現在，一種“perforin”單聚物（小鼠perforin R213E）的結構已被確定。對該結構所做分析同時結合對低聚孔的一個冷電子顯微鏡重建結果表明，這個孔內的“perforin”單聚物與依賴于的溶細胞素在結構上同源的單聚物相比采用一種“內面向外”的取向。
這種新穎的適應性也許可解釋“perforin”是怎樣將支持細胞凋亡的蛋白酶（granzymes）送入目標細胞中的以及相關的互補免疫蛋白是怎樣組裝成微孔的。
The structural basis for membrane binding and pore formation by lymphocyte perforin
Ruby H. P. Law,Natalya Lukoyanova,Ilia Voskoboinik,Tom T. Caradoc-Davies,Katherine Baran,Michelle A. Dunstone,Michael E. D’Angelo,Elena V. Orlova,Fasséli Coulibaly,Sandra Verschoor,Kylie A. Browne,Annette Ciccone,Michael J. Kuiper,Phillip I. Bird,Joseph A. Trapani,joe.trapani@Helen R. Saibil& James C. Whisstock
Natural killer cells and cytotoxic T lymphocytes accomplish the critically important function of killing virus-infected and neoplastic cells. They do this by releasing the pore-forming protein perforin and granzyme proteases from cytoplasmic granules into the cleft formed between the abutting killer and target cell membranes. Perforin, a 67-kilodalton multidomain protein, oligomerizes to form pores that deliver the pro-apoptopic granzymes into the cytosol of the target cell1, 2, 3, 4, 5, 6. The importance of perforin is highlighted by the fatal consequences of congenital perforin deficiency, with more than 50 different perforin mutations linked to familial haemophagocytic lymphohistiocytosis （type 2 FHL）7. Here we elucidate the mechanism of perforin pore formation by determining the X-ray crystal structure of monomeric murine perforin, together with a cryo-electron microscopy reconstruction of the entire perforin pore. Perforin is a thin ‘key-shaped’ molecule, comprising an amino-terminal membrane attack complex perforin-like （MACPF）/cholesterol dependent cytolysin （CDC） domain8, 9 followed by an epidermal growth factor （EGF） domain that, together with the extreme carboxy-terminal sequence, forms a central shelf-like structure. A C-terminal C2 domain mediates initial, Ca2+-dependent membrane binding. Most unexpectedly, however, electron microscopy reveals that the orientation of the perforin MACPF domain in the pore is inside-out relative to the subunit arrangement in CDCs10, 11. These data reveal remarkable flexibility in the mechanism of action of the conserved MACPF/CDC fold and provide new insights into how related immune defence molecules such as complement proteins assemble into pores.
