Required for both basal and inducible autophagy (PubMed:19270696, PubMed:19270693). Determines the localization of the autophagy-specific PI3-kinase complex PI3KC3-C1 (By similarity). Plays a role in autophagosome formation and MAP1LC3/LC3 conjugation to phosphatidylethanolamine (PubMed:19270696, PubMed:19270693). Promotes BECN1 translocation from the trans-Golgi network to autophagosomes (By similarity). Enhances PIK3C3 activity in a BECN1-dependent manner. Essential for the autophagy-dependent phosphorylation of BECN1 (By similarity). Stimulates the phosphorylation of BECN1, but suppresses the phosphorylation of PIK3C3 by AMPK (PubMed:23332761). Binds to STX17-SNAP29 binary t-SNARE complex on autophagosomes and primes it for VAMP8 interaction to promote autophagosome-endolysosome fusion (By similarity). Modulates the hepatic lipid metabolism (PubMed:22992773). Forms homooligomers; homo-oligomerization is essential for the roles in membrane tethering and enhancement of SNARE-mediated fusion (By similarity). 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:19270693, PubMed:22745922, PubMed:23332761). PI3KC3-C1 displays a V-shaped architecture with PIK3R4 serving as a bridge between PIK3C3 and the ATG14:BECN1 subcomplex (By similarity). PI3KC3-C1 can associate with further regulatory subunits (PubMed:24849286). Interacts with PIK3CB (PubMed:21059846). Interacts (via coiled-coil domain) with BECN2 (via coiled-coil domain); this interaction is tighter than BECN2 self-association (By similarity). Interacts with the STX17-SNAP29 binary t-SNARE complex (By similarity). Interacts with NRBF2 (PubMed:24849286) Interacts with PIK3C3 and BECN1; this interaction is increased in the absence of TMEM39A (By similarity). Interacts with STEEP1; the interaction is required for trafficking of STING1 from the endoplasmic reticulum (By similarity). Cytosolic under nutrient-rich conditions (PubMed:20639694). Following autophagy stimuli, such as starvation or rapamycin induction, predominantly detected in cytoplasmic foci, identified as isolation membranes and autophagosomes (PubMed:20639694). Accumulates on highly curved PtdIns(3)P enriched autophagic membrane via its BATS domain to sense and maintain membrane curvature (PubMed:21518905). Localizes also to discrete punctae along the ciliary axoneme and to the base of the ciliary axoneme (PubMed:24089209). Widely expressed. Expression is controlled by forkhead box O FoxO1 transcription factor and circadian rhythms. 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.