Ubiquitin-like modifier that increases cell-surface expression of kappa-type opioid receptor through facilitating anterograde intracellular trafficking of the receptor. Involved in formation of autophagosomal vacuoles. While LC3s are involved in elongation of the phagophore membrane, the GABARAP/GATE-16 subfamily is essential for a later stage in autophagosome maturation. 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. Interacts with ATG13, OPRK1, RB1CC1 and ULK1. Interacts with TP53INP1 and TP53INP2. Directly interacts with SQSTM1. Interacts with ATG3, ATG7 and MAP15. Interacts with TECPR2. Interacts with TBC1D5. Interacts with MAPK15. Interacts with TRIM5. Interacts with MEFV and TRIM21. Interacts with WDFY3. Interacts with the reticulophagy receptor TEX264. Interacts with UBA5. Interacts with KBTBD6 AND KBTBD7; the interaction is direct (By similarity). High expression in liver and brain, lower in lung and heart and very low in kidney and skeletal muscle (at protein level). Present in all brain regions and spinal cord (at protein level). 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, GABARAPL1-I. The processed form is then activated by APG7L/ATG7, transferred to ATG3 and conjugated to phosphatidylethanolamine (PE) phospholipid to form the membrane-bound form, GABARAPL1-II. During non-canonical autophagy, the processed form is conjugated to phosphatidylserine (PS) phospholipid. ATG4 proteins also mediate the delipidation of PE-conjugated forms required for GABARAPL1 recycling when autophagosomes fuse with lysosomes. 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). Belongs to the ATG8 family.