Plays a central role in autophagy (PubMed:18570871, PubMed:21358617, PubMed:23184933, PubMed:23974797, PubMed:28445460). Acts as core subunit of the PI3K complex that mediates formation of phosphatidylinositol 3-phosphate; different complex forms are believed to play a role in multiple membrane trafficking pathways: PI3KC3-C1 is involved in initiation of autophagosomes and PI3KC3-C2 in maturation of autophagosomes and endocytosis. Involved in regulation of degradative endocytic trafficking and required for the abcission step in cytokinesis, probably in the context of PI3KC3-C2 (PubMed:20643123, PubMed:20208530, PubMed:23974797, PubMed:26783301). Essential for the formation of PI3KC3-C2 but not PI3KC3-C1 PI3K complex forms. Involved in endocytosis (PubMed:25275521). Protects against infection by a neurovirulent strain of Sindbis virus (PubMed:9765397). May play a role in antiviral host defense. Beclin-1-C 35 kDa localized to mitochondria can promote apoptosis; it induces the mitochondrial translocation of BAX and the release of proapoptotic factors. A homodimeric form is proposed to exist; this metastable form readily transits to ATG14- or UVRAG-containing complexes with BECN1:UVRAG being more stable than BECN1:ATG14 (By similarity). Component of the PI3K (PI3KC3/PI3K-III/class III phosphatidylinositol 3-kinase) complex the core of which is composed of the catalytic subunit PIK3C3, the regulatory subunit PIK3R4 and BECN1 associating with additional regulatory/auxilliary subunits to form alternative complex forms. Alternative complex forms containing a forth regulatory subunit in a mutually exclusive manner are PI3K complex I (PI3KC3-C1) containing ATG14, and PI3K complex II (PI3KC3-C2) containing UVRAG. PI3KC3-C1 displays a V-shaped architecture with PIK3R4 serving as a bridge between PIK3C3 and the ATG14:BECN1 subcomplex (PubMed:18843052, PubMed:19050071, PubMed:19270696, PubMed:23878393, PubMed:25490155). Both, PI3KC3-C1 and PI3KC3-C2, can associate with further regulatory subunits, such as RUBCN, SH3GLB1/Bif-1 and AMBRA1 (PubMed:20643123, PubMed:19270696). PI3KC3-C1 probably associates with PIK3CB (By similarity). Interacts with AMBRA1, GOPC, GRID2 (PubMed:21358617). Forms a complex with PPP2CA and AMBRA1; AMBRA1 and BECN1 components of the complex regulate MYC stability via different pathways (PubMed:25803737). Interacts with BCL2 and BCL2L1 isoform Bcl-X(L); the interaction inhibits BECN1 function in promoting autophagy by interfering with the formation of the PI3K complex (PubMed:9765397, PubMed:16179260, PubMed:17446862, PubMed:18570871, PubMed:21358617, PubMed:22498477, PubMed:17337444, PubMed:17659302). Interacts with cytosolic HMGB1; inhibits the interaction of BECN1 and BCL2 leading to promotion of autophagy (PubMed:20819940). Interacts with USP10, USP13, VMP1, DAPK1, RAB39A (PubMed:19180116, PubMed:17724469, PubMed:17337444, PubMed:21962518, PubMed:24349490). Interacts with the poly-Gln domain of ATXN3; the interaction causes deubiquitination at Lys-402 and stabilizes BECN1 (PubMed:28445460). Interacts with SLAMF1 (PubMed:22493499). Interacts with TRIM5; the interaction causes activation of BECN1 by causing its dissociation from its inhibitors BCL2 and TAB2 (PubMed:25127057). Interacts with active ULK1 (phosphorylated on 'Ser-317') and MEFV simultaneously (PubMed:26347139). Interacts with WDR81 and WDR91; negatively regulates the PI3 kinase/PI3K activity associated with endosomal membranes (PubMed:26783301). Interacts with LAPTM4B; competes with EGFR for LAPTM4B binding; regulates EGFR activity (PubMed:28479384). Interacts with TRIM50 (PubMed:29604308). Interacts with TRIM16. Interacts with ATG14; this interaction is increased in the absence of TMEM39A (PubMed:31806350). Interacts with WASHC1; preventing interaction with AMBRA1 and the DCX(AMBRA1) complex and subsequent ubiquitination (PubMed:23974797). Interacts with TRIM17 (PubMed:27562068). Interacts with BCL2L10/BCL-B (via BH1 domain) (PubMed:22498477). (Microbial infection) Interacts with human cytomegalovirus/HHV-5 protein TRS1. (Microbial infection) Interacts with murine gammaherpesvirus 68 M11. (Microbial infection) Interacts with herpes simplex virus 1 (HHV-1) protein ICP34.5; this interaction antagonizes the host autophagy response. Interaction with ATG14 promotes translocation to autophagosomes. Expressed in dendrites and cell bodies of cerebellar Purkinje cells (By similarity). Ubiquitous. The coiled coil domain can form antiparallel homodimers and mediates dimerization with the coiled coil domains of ATG14 or UVRAG involved in the formation of PI3K complexes. The C-terminal evolutionary conserved domain (ECD) contains poly-Gln-binding domains such as the ATXN3 poly-Gln motif, consistent with structural docking models revealing two highly scored poly-Gln-binding pockets in the ECD (PubMed:28445460). As some binding is observed with BECN1 lacking the ECD, other domains of BECN1 may also interact with ATXN3 (PubMed:28445460). Phosphorylation at Thr-119 by DAPK1 reduces its interaction with BCL2 and BCL2L1 and promotes induction of autophagy (PubMed:19180116). In response to autophagic stimuli, phosphorylated at serine residues by AMPK in an ATG14-dependent manner, and this phosphorylation is critical for maximally efficient autophagy (PubMed:23878393). Polyubiquitinated by NEDD4, both with 'Lys-11'- and 'Lys-63'-linkages (PubMed:21936852). 'Lys-11'-linked polyubiquitination leads to degradation and is enhanced when the stabilizing interaction partner VPS34 is depleted (PubMed:21936852). Deubiquitinated by USP10 and USP13, leading to stabilize the PIK3C3/VPS34-containing complexes (PubMed:21962518). Polyubiquitinated at Lys-402 with 'Lys-48'-linkages (PubMed:28445460). 'Lys-48'-linked polyubiquitination of Lys-402 leads to degradation (PubMed:28445460). Deubiquitinated by ATXN3, leading to stabilization (PubMed:28445460). Ubiquitinated at Lys-437 via 'Lys-63'-linkage by the DCX(AMBRA1) complex, thereby increasing the association between BECN1 and PIK3C3 to promote PIK3C3 activity (PubMed:23974797). Proteolytically processed by caspases including CASP8 and CASP3; the C-terminal fragments lack autophagy-inducing capacity and are proposed to induce apoptosis. Thus the cleavage is proposed to be an determinant to switch from autophagy to apoptosis pathways affecting cellular homeostasis including viral infections and survival of tumor cells. Expanded poly-Gln tracts inhibit ATXN3-BECN1 interaction, decrease BECN1 levels and impair starvation-induced autophagy (PubMed:28445460). Belongs to the beclin family.