SEPT11, a Novel Platelet Septin Partners with the Platelet Septin SEPT5.

The septin SEPT11 is a novel member of the highly conserved septin family. Septins are GTP-binding proteins which form heteropolymeric complexes. In non-dividing cells (such as platelets and neurons) septins are implicated in exocytosis. The septins SEPT4, SEPT5 and SEPT8 are expressed in platelets. Platelets from a SEPT5 knockout mouse showed an altered serotonin secretion and platelet aggregation suggesting that SEPT5 is involved in secretion in platelets. Transmission electron microscopy of platelets revealed that SEPT4, SEPT5 and SEPT8 are localized surrounding alpha-granules suggesting that the three septins may be components of the septin complex in platelets and contribute in such a way to platelet biology. Activation of platelets by agonists resulted in the translocation of SEPT4 and SEPT8 to the platelet surface indicating a possible functional role of these proteins in granular secretion. Previously, we had identified the interaction of the septins SEPT5 and SEPT8. The aim of this study was to identify other interaction partners of the human septin SEPT5. Using the yeast two-hybrid system we now demonstrate that SEPT11 partners with SEPT5. Western analysis revealed that SEPT11 is also expressed in platelets. The molecular interaction of SEPT11 with SEPT5 was verified by precipitation of the SEPT5/SEPT11 complex from lysates of human platelets. In addition, using Western analysis and immunofluorescence analysis, the co-expression of SEPT5 and SEPT11 is shown in human endothelial cells (HUVECs). The Western analyses of various mouse tissues showed that the expression pattern of SEPT11 matches with the expression pattern of SEPT5 suggesting that SEPT11 and SEPT5 are components of the corresponding cell specific septin complex. Since SEPT5 and SEPT11 are members of the same septin complex and since SEPT5 has been reported to play an important role in exocytosis, the SEPT5/SEPT11-complex may be involved in exocytosis in platelets and HUVECs.