Upon binding L-arginine, SLC38A9 acts as a guanyl nucleotide exchange factor (GEF) that enhances the conversion of RRAGA:GDP (RagA:GDP, and by inference RRAGB:GDP, RagB:GDP) to RRAGA:GTP, the active form (Shen and Sabatini 2018). The Ragulator complex also shows GEF activity for RRAGA,B (Bar-Peled et al. 2012). SH3BP4 inhibits loading of GTP onto RRAGB during amino acid starvation (Kim et al. 2012). The binding of GTP by RRAGA,B appears to inhibit binding of GTP by RRAGC,D (Shen et al. 2017).