As a cytolytic mediator, Perforin is released from the cytoplasmic granules into the immune synapse formed between Natural Killer cells/cytotoxic T-lymphocytes and target cells such as virually-infected cells/transformed cells. Upon binding to the target cells, perforin oligomerises to form pores that permit the delivery of pro-apototic granzymes into the target cells, hence cytolysis. The mechanism through which perforin switches from a monomeric soluble form to a oligomeric membrane bound form (through major rearrangements of the protein fold) is poorly understood. Here we use x-ray crystallography to sample the conformational variability of the perforin fold. We observed that perforin monomer is a thin “key-shaped” molecule comprising of three structural domains: an N-terminal MACPF/Cholesterol Dependent Cytolysin (CDC) domain, a flexible central shelf-like structure consisting of an Epidermal Growth Factor domain, and the extreme C-terminal sequence and a type II calcium binding C2 domain. Interestingly, we observed that the key-shaped structure exists in multiple conformations and is in agreement with the cryo-EM studies previously published.
Law et al., 2010. Nature 468: 447-451