Acts as a positive activator of the GCN2 protein kinase activity in response to in response to low amino acid, carbon, or purine availability (PubMed:7621831, PubMed:10733573). Component of the GCN1-GCN20 complex that forms a complex with GCN2 on translating ribosomes; during this process, GCN20 helps GCN1 to act as a chaperone to facilitate delivery of uncharged tRNAs that enter the A site of ribosomes to the tRNA-binding domain of GCN2, and hence stimulating GCN2 kinase activity (PubMed:7621831, PubMed:9234705, PubMed:10775272, PubMed:15722345). Participates in gene-specific mRNA translation activation, such as the transcriptional activator GCN4, by promoting the GCN2-mediated phosphorylation of eukaryotic translation initiation factor 2 (eIF-2-alpha/SUI2) on 'Ser-52', and hence allowing GCN4-mediated reprogramming of amino acid biosynthetic gene expression to alleviate nutrient depletion (PubMed:15722345). Interacts (via N-terminus) with GCN1 (via C-terminus); this interaction stimulates GCN2 kinase activity in response to amino acid starvation (PubMed:7621831, PubMed:9234705, PubMed:11101534, PubMed:15722345). The GCN1-GCN20 complex interacts with GCN2 on translating ribosomes in amino acid-starved cells; this association stimulates GCN2 kinase activation by uncharged tRNAs, and hence allowing GCN4 translational activation and derepression of amino acid biosynthetic genes (PubMed:7621831, PubMed:9234705, PubMed:10775272, PubMed:11101534). Associates with ribosomes (PubMed:7621831, PubMed:9234705, PubMed:15722345). Inhibits GCN4 derepression in amino acid or purine-starved cells (PubMed:10733573). Attenuates GCN4 derepression in glucose-starved cells (PubMed:10733573). Present with 14600 molecules/cell in log phase SD medium. Belongs to the ABC transporter superfamily. ABCF family. EF3 subfamily.