Filament-forming cytoskeletal GTPase (By similarity). May play a role in cytokinesis (Potential). May play a role in the internalization of 2 intracellular microbial pathogens, Listeria monocytogenes and Shigella flexneri. Septins polymerize into heterooligomeric protein complexes that form filaments, and associate with cellular membranes, actin filaments, and microtubules. GTPase activity is required for filament formation. Interacts with SEPTIN2, SEPTIN6, SEPTIN7, SEPTIN11 and SEPTIN14. Interacts with RTKN and ARHGEF18. In a mesenchymal cell line, Rho/RTKN signals cause disruption of wild-type septin filaments, but not of those containing isoform 2 variants HNA Trp-106 and Phe-111. In a mesenchymal cell line, isoform 2 variants HNA Trp-106 and Phe-111, but not wild type, form filaments with SEPTIN4. In an epithelial cell line, concentrates at cell-cell contact areas. After TGF-beta1 treatment and induction of epithelial to mesenchymal transition, colocalizes partly with actin stress fibers. During bacterial infection, displays a collar shape structure next to actin at the pole of invading bacteria. There are potentially 18 isoforms. Widely expressed. Isoforms are differentially expressed in testes, kidney, liver heart, spleen, brain, peripheral blood leukocytes, skeletal muscle and kidney. Specific isoforms appear to demonstrate tissue specificity. Isoform 5 is the most highly expressed in fetal tissue. Isoform 1 is detected in all tissues except the brain and thymus, while isoform 2, isoform 3, and isoform 4 are detected at low levels in approximately half of the fetal tissues. A chromosomal aberration involving SEPTIN9/MSF is found in therapy-related acute myeloid leukemia (t-AML). Translocation t(11;17)(q23;q25) with KMT2A/MLL1. The disease is caused by variants affecting the gene represented in this entry. Belongs to the TRAFAC class TrmE-Era-EngA-EngB-Septin-like GTPase superfamily. Septin GTPase family. Aberrant splicing.