Substrate-recognition component of a cullin-5-RING E3 ubiquitin-protein ligase complex (ECS complex, also named CRL5 complex), which mediates the ubiquitination and subsequent proteasomal degradation of target proteins, such as CGAS and SNAI1 (PubMed:29059170, PubMed:38418882). The ECS(SPSB3) complex catalyzes 'Lys-48'-linked ubiquitination of nuclear CGAS in cycling cells, leading to its degradation (PubMed:38418882). Recognizes and binds nucleosome-bound CGAS: ubiquitination and degradation of nuclear CGAS during G1 and G2 phases is required to promote low intranuclear CGAS abundance before the next mitotic cycle (PubMed:38418882). The ECS(SPSB3) complex also mediates ubiquitination and degradation of phosphorylated SNAI1 (PubMed:29059170). {ECO:0000269|PubMed:29059170, ECO:0000269|PubMed:38418882}. This is the function of Ensembl gene identifier ENSG00000162032 (SPSB3, splA/ryanodine receptor domain and SOCS box containing 3).