Functions as an intracellular leucine sensor that negatively regulates the TORC1 signaling pathway through the GATOR complex (PubMed:18692468, PubMed:25263562, PubMed:25457612, PubMed:26449471, PubMed:26612684, PubMed:26586190, PubMed:31586034, PubMed:35114100). In absence of leucine, binds the GATOR subcomplex GATOR2 and prevents TORC1 signaling (PubMed:18692468, PubMed:25263562, PubMed:25457612, PubMed:26449471, PubMed:26612684, PubMed:26586190, PubMed:31586034, PubMed:35114100). Binding of leucine to SESN2 disrupts its interaction with GATOR2 thereby activating the TORC1 signaling pathway (PubMed:26449471, PubMed:26586190, PubMed:35114100). This stress-inducible metabolic regulator also plays a role in protection against oxidative and genotoxic stresses. May negatively regulate protein translation in response to endoplasmic reticulum stress, via TORC1 (PubMed:24947615). May positively regulate the transcription by NFE2L2 of genes involved in the response to oxidative stress by facilitating the SQSTM1-mediated autophagic degradation of KEAP1 (PubMed:23274085). May also mediate TP53 inhibition of TORC1 signaling upon genotoxic stress (PubMed:18692468). Moreover, may prevent the accumulation of reactive oxygen species (ROS) through the alkylhydroperoxide reductase activity born by the N-terminal domain of the protein (PubMed:26612684). Was originally reported to contribute to oxidative stress resistance by reducing PRDX1 (PubMed:15105503). However, this could not be confirmed (PubMed:19113821). a hydroperoxide + L-cysteinyl-[protein] = an alcohol + S-hydroxy-L-cysteinyl-[protein] Interacts with the GATOR2 complex which is composed of MIOS, SEC13, SEH1L, WDR24 and WDR59; the interaction is negatively regulated by leucine (PubMed:25263562, PubMed:25457612, PubMed:26449471, PubMed:35114100). Interacts with RRAGA, RRAGB, RRAGC and RRAGD; may function as a guanine nucleotide dissociation inhibitor for RRAGs and regulate them (PubMed:25259925). May interact with the TORC2 complex (By similarity). Interacts with KEAP1, RBX1, SQSTM and ULK1; to regulate the degradation of KEAP1 (PubMed:23274085, PubMed:25040165). May also associate with the complex composed of TSC1, TSC2 and the AMP-responsive protein kinase/AMPK to regulate TORC1 signaling (PubMed:18692468). May interact with PRDX1 (PubMed:15105503). Widely expressed. Up-regulated by hypoxia and DNA damage (PubMed:12203114). Up-regulated by treatments inducing endoplasmic reticulum stress (PubMed:24947615). The N-terminal domain has an alkylhydroperoxide reductase activity. The C-terminal domain mediates interaction with GATOR2 through which it regulates TORC1 signaling. Phosphorylated by ULK1 at multiple sites. Ubiquitinated at Lys-175 by RNF167 via 'Lys-63'-linked polyubiquitination in response to leucine deprivation: ubiquitination promotes SESN2-interaction with the GATOR2 complex, leading to inhibit the TORC1 signaling pathway (PubMed:35114100). Deubiquitinated at Lys-175 by STAMBPL1, promoting the TORC1 signaling pathway (PubMed:35114100). Ubiquitinated by RNF186; ubiquitination mediates proteasomal degradation (PubMed:31586034). Belongs to the sestrin family.