Platelet septin complexes form rings and associate with the microtubular network

Background: Septins are important effectors in molecular mechanisms involving membrane partitioning. To date, a growing repertoire of septins in mammals includes 13 different proteins (SEPT1 to SEPT13) that can be classified into four distinct categories based on sequence similarity. Aim: In this study, we document the human platelet septin, SEPT5, as part of a complex composed of multiple septin proteins. Results: Biochemical and immunofluorescent data place the majority of these complexes in the platelet periphery as part of the platelet circumferential band copurifying with the platelet microtubule coil and tubulin. The presence of a prominent platelet septin ring in resting platelets appears to be left intact in the activated platelet, as a similar ring structure is observed following platelet spreading on fibrinogen. The ablation of SEPT5 in the knock-out mouse model had previously been reported to result in a platelet phenotype with aggregation using subthreshold levels of agonist. Speculation on the role of SEPT5 in the platelet-release reaction suggested that SEPT5 regulates platelet function by association with platelet storage granules. We now report that the absence of SEPT5 results in increased ATP release from stimulated platelets. Conclusion: These studies document the presence of platelet septin complexes and validate the importance of septins for platelet physiology.