Ubiquitin-like modifier involved in formation of autophagosomal vacuoles (autophagosomes). Plays a role in mitophagy which contributes to regulate mitochondrial quantity and quality by eliminating the mitochondria to a basal level to fulfill cellular energy requirements and preventing excess ROS production. In response to cellular stress and upon mitochondria fission, binds C-18 ceramides and anchors autophagolysosomes to outer mitochondrial membranes to eliminate damaged mitochondria. While LC3s are involved in elongation of the phagophore membrane, the GABARAP/GATE-16 subfamily is essential for a later stage in autophagosome maturation. Promotes primary ciliogenesis by removing OFD1 from centriolar satellites via the autophagic pathway. Through its interaction with the reticulophagy receptor TEX264, participates in the remodeling of subdomains of the endoplasmic reticulum into autophagosomes upon nutrient stress, which then fuse with lysosomes for endoplasmic reticulum turnover. Upon nutrient stress, directly recruits cofactor JMY to the phagophore membrane surfaces and promotes JMY's actin nucleation activity and autophagosome biogenesis during autophagy. 3 different light chains, LC1, LC2 and LC3, can associate with MAP1A and MAP1B proteins (PubMed:7908909). Interacts at microtubules with CABP1 (via EF-hands 1 and 2) but not with calmodulin (PubMed:15095872). Interacts with FYCO1 (via C-terminus). Interacts with TP53INP1 and TP53INP2. Interacts with TBC1D25. Directly interacts with SQSTM1; this interaction leads to MAP1LC3B recruitment to inclusion bodies containing polyubiquitinated protein aggregates and to inclusion body degradation by autophagy (By similarity). Interacts with ATG4B (PubMed:19322194). Interacts with MAPK15 and BNIP3. Interacts with MAPB1, KEAP1, PCM1, OFD1, CEP131, and TECPR2. Interacts with TBC1D5. Found in a complex with UBQLN1 and UBQLN2. Interacts with UBQLN4 (via STI1 1 and 2 domains). Interacts with UBQLN1 in the presence of UBQLN4. Interacts with ATG13. Interacts with RETREG1, RETREG2 and RETREG3 (By similarity). Interacts with PLCL1; the interaction inhibits autophagosome formation (PubMed:23399561). Interacts with TRIM16 (By similarity). Interacts with CRY1 and PER2 (By similarity). Interacts with the reticulophagy receptor TEX264 (By similarity). Membrane-bound form LC3-II interacts with PHB1 and PHB2; the interaction takes place upon Parkin-mediated mitochondrial damage (By similarity). Interacts with PJVK; the interaction is direct (By similarity). Interacts with KBTBD6 and KBTBD7; the interaction is direct (By similarity). Interacts with AMBRA1 (via LIR motif) (By similarity). Interacts with JMY; the interaction results in the activation of JYM's nucleation activity in the cytoplasm (By similarity). Interacts with MOAP1 (via LIR motif) (By similarity). LC3-II binds to the autophagic membranes. LC3-II localizes with the mitochondrial inner membrane during Parkin-mediated mitophagy (By similarity). Localizes also to discrete punctae along the ciliary axoneme (By similarity). Abundant only in neurons. Detected in testes. The precursor molecule is cleaved by ATG4 (ATG4A, ATG4B, ATG4C or ATG4D) to expose the glycine at the C-terminus and form the cytosolic form, LC3-I (PubMed:11060023, PubMed:16183633, PubMed:16874114). The processed form is then activated by APG7L/ATG7, transferred to ATG3 and conjugated to phosphatidylethanolamine (PE) phospholipid to form the membrane-bound form, LC3-II. During non-canonical autophagy, the processed form is conjugated to phosphatidylserine (PS) phospholipid. ATG4 proteins also mediate the delipidation of PE-conjugated forms. In addition, ATG4B and ATG4D mediate delipidation of ATG8 proteins conjugated to PS during non-canonical autophagy. ATG4B constitutes the major protein for proteolytic activation (By similarity). ATG4D is the main enzyme for delipidation activity (By similarity). Phosphorylation by PKA inhibits conjugation of phosphatidylethanolamine (PE). Interaction with MAPK15 reduces the inhibitory phosphorylation and increases autophagy activity. Belongs to the ATG8 family.