Plays a central role in autophagy (PubMed:24441423, PubMed:27467399, PubMed:28445460). Acts as core subunit of different PI3K complex forms that mediate formation of phosphatidylinositol 3-phosphate and 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 (By similarity). Involved in regulation of degradative endocytic trafficking and required for the abcission step in cytokinesis, probably in the context of PI3KC3-C2 (By similarity). Essential for the formation of PI3KC3-C2 but not PI3KC3-C1 PI3K complex forms (By similarity). Involved in endocytosis including endosome formation in neuronal cells (By similarity). Component of the PI3K (PI3KC3/PI3K-III/class III phosphatidylinositol 3-kinase) complex (By similarity). Interacts with the poly-Gln domain of ATXN3; the interaction causes deubiquitination at Lys-399 and stabilizes BECN1 (PubMed:28445460). Expressed during follicle development in the ovary (PubMed:23195281). 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. As some binding is observed with BECN1 lacking the ECD, other domains of BECN1 may also interact with ATXN3. Polyubiquitinated at Lys-399 with 'Lys-48'-linkages (PubMed:28445460). 'Lys-48'-linked polyubiquitination of Lys-399 leads to degradation (PubMed:28445460). Deubiquitinated by ATXN3, leading to stabilization (PubMed:28445460). Results in defects in morphogenesis, increased numbers of dead cells, abnormal heart structure, and reduced organismal survival (PubMed:24441423). Reduces the regenerative response to muscle injury with accumulation of sarcomeric and nuclear debris (PubMed:27467399). Expanded poly-Gln tracts inhibit ATXN3-BECN1 interaction, decrease BECN1 levels and impair starvation-induced autophagy (PubMed:28445460). Belongs to the beclin family.