SEPTIN3

SEPTIN3 is a brain-enriched, GTP-binding septin subunit that assembles into heteromeric filaments and participates in neuronal membrane-associated processes (PMID:17564677, PMID:24648497). It directly binds SEPT5 and SEPT7 to form a heteromeric complex localized to nerve terminals adjacent to synaptic vesicle compartments, and its ability to form filaments in cells depends on its GTP- and phosphoinositide-binding domains (PMID:17564677). As a member of the SEPT3 subgroup (with SEPT9 and SEPT12), it serves as a cognate subunit specifically in heterooctameric—rather than hexameric—septin complexes, so that tissue-specific expression of subgroup members dictates whether octamers or atypical tetramers are assembled, shaping higher-order filament properties (PMID:24648497). Crystallographic comparison across the SEPT3 subgroup reveals plasticity at the NC filament interface in which nucleotide-state changes expel a polybasic region that is otherwise sequestered by a polyacidic region, coupling GTP binding/hydrolysis to interaction with negatively charged membranes (PMID:32431830). In neurons, SEPT3 binds the autophagy proteins LC3B and GABARAPL2, increases its colocalization with LC3B upon autophagy induction, localizes to LC3B-positive membranes and mitochondria, and accumulates with autophagy proteins upon autophagy modulation, implicating it in synaptic autophagy (PMID:35932293). SEPT3 is dispensable for normal neuronal development, as knockout mice show no histogenesis or differentiation defects (PMID:17564677).

1. 2007 Medium

   Established that SEPT3 is not an isolated protein but a direct binding partner of other septins, defining its place in a neuronal heteromeric complex at synaptic terminals.

   Evidence Co-immunoprecipitation, domain-mutant overexpression in HEK293 cells, and immunofluorescence in primary hippocampal neurons

   PMID:17564677

   Open questions at the time

   • Stoichiometry and architecture of the SEPT3-SEPT5-SEPT7 complex not resolved
   • Functional consequence of complex assembly at terminals untested
2. 2007 Medium

   Tested whether SEPT3 is required for nervous system development; loss-of-function showed it is dispensable, redirecting attention to non-developmental roles.

   Evidence Sept3 knockout mice with histology and Western blotting of synaptic proteins and other septins

   PMID:17564677

   Open questions at the time

   • Possible functional redundancy with other septins not excluded
   • Behavioral or synaptic-function phenotypes not assessed
3. 2014 High

   Defined the precise role of SEPT3 in septin assembly: it is a cognate octamer-specific subunit whose expression governs filament composition, explaining how tissue context tunes septin architecture.

   Evidence Native complex analysis in genetically manipulated human cells with size-exclusion chromatography and immunoblotting

   PMID:24648497

   Open questions at the time

   • Functional difference between octamer- and tetramer-based filaments not defined
   • In vivo relevance of composition control not tested
4. 2020 High

   Provided the structural mechanism by which the SEPT3 subgroup couples nucleotide state to membrane binding, explaining the subgroup's distinctive membrane-association behavior.

   Evidence X-ray crystallography of SEPT3-subgroup GTP-binding domains in GDP and GTPgammaS states

   PMID:32431830

   Open questions at the time

   • Squeezing/polybasic mechanism directly demonstrated for SEPT9, inferred for SEPT3
   • Membrane association in cells not directly tested from these structures
5. 2022 Medium

   Connected SEPT3 to autophagy by identifying direct binding to Atg8-family proteins and localization to autophagic membranes in neurons, implicating it in synaptic autophagy.

   Evidence Co-immunoprecipitation, immunofluorescence with chemical autophagy modulation, and electron microscopy in primary neurons

   PMID:35932293

   Open questions at the time

   • Functional requirement of SEPT3 for autophagic flux not established
   • LIR motif or binding interface with LC3B/GABARAPL2 not mapped
6. 2022 Low

   Tested whether SEPT3 mitochondrial localization reflects a mitophagy role; lack of PINK1 colocalization argued against a mitophagy-specific function.

   Evidence Immunofluorescence colocalization with PINK1 in primary neurons

   PMID:35932293

   Open questions at the time

   • Single method, single marker negative result
   • Alternative mitochondrial functions not explored
7. 2023 Low

   Examined a possible role for SEPT3 in cancer, linking its expression to TNBC aggressiveness through Wnt pathway activation.

   Evidence Lentiviral knockdown/overexpression in TNBC cell lines, functional assays, xenograft, and Western blotting of Wnt components

   PMID:37720177

   Open questions at the time

   • Pathway placement inferred from Western blotting without direct binding or epistasis
   • No mechanistic reconstitution of SEPT3-Wnt link

• How GTP-state-dependent membrane association and Atg8-family binding are integrated into a defined function of SEPT3 in synaptic autophagy remains unresolved.
• No loss-of-function autophagy phenotype demonstrated for SEPT3
• Recruitment mechanism linking septin filaments to autophagic membranes unknown
• Physiological substrate or cargo of SEPT3-associated autophagy undefined