Required for both basal and inducible autophagy. Determines the localization of the autophagy-specific PI3-kinase complex PI3KC3-C1 (PubMed:18843052, PubMed:19050071). Plays a role in autophagosome formation and MAP1LC3/LC3 conjugation to phosphatidylethanolamine (PubMed:19270696, PubMed:20713597). Promotes BECN1 translocation from the trans-Golgi network to autophagosomes (PubMed:20713597). Enhances PIK3C3 activity in a BECN1-dependent manner. Essential for the autophagy-dependent phosphorylation of BECN1 (PubMed:23878393). Stimulates the phosphorylation of BECN1, but suppresses the phosphorylation PIK3C3 by AMPK (PubMed:23878393). Binds to STX17-SNAP29 binary t-SNARE complex on autophagosomes and primes it for VAMP8 interaction to promote autophagosome-endolysosome fusion (PubMed:25686604). Modulates the hepatic lipid metabolism (By similarity). Forms homooligomers; homo-oligomerization is essential for the roles in membrane tethering and enhancement of SNARE-mediated fusion (PubMed:25686604). Component of the PI3K (PI3KC3/PI3K-III/class III phosphatidylinositol 3-kinase) complex I (PI3KC3-C1) in which the core composed of the catalytic subunit PIK3C3, the regulatory subunit PIK3R4 and BECN1 is associated with ATG14 (PubMed:18843052, PubMed:19050071, PubMed:19270696, PubMed:22314358, PubMed:23878393). PI3KC3-C1 displays a V-shaped architecture with PIK3R4 serving as a bridge between PIK3C3 and the ATG14:BECN1 subcomplex (PubMed:25490155). PI3KC3-C1 can associate with further regulatory subunits. Interacts with PIK3CB (By similarity). Interacts (via coiled-coil domain) with BECN2 (via coiled-coil domain); this interaction is tighter than BECN2 self-association (PubMed:23954414, PubMed:28218432). Interacts with the STX17-SNAP29 binary t-SNARE complex (PubMed:25686604). Interacts with NRBF2 (By similarity). Interacts with PIK3C3 and BECN1; this interaction is increased in the absence of TMEM39A (PubMed:31806350). Interacts with STEEP1; the interaction is required for trafficking of STING1 from the endoplasmic reticulum (PubMed:32690950). Cytosolic under nutrient-rich conditions (PubMed:19050071). Following autophagy stimuli, such as starvation or rapamycin induction, predominantly detected in cytoplasmic foci, identified as isolation membranes and autophagosomes (PubMed:19050071). Accumulates on highly curved PtdIns(3)P enriched autophagic membrane via its BATS domain to sense and maintain membrane curvature (By similarity). Localizes also to discrete punctae along the ciliary axoneme and to the base of the ciliary axoneme (By similarity). The coiled-coil domain is required for BECN1- and PIK3C3-binding and for autophagy. The final 80 residues in the C-terminus define a minimum required region for autophagosome binding called BATS. The N-terminal cysteine repeats are required for proper localization to the endoplasmic reticulum. Belongs to the ATG14 family. Extended N-terminus.