Substrate-specific adapter of a BCR (BTB-CUL3-RBX1) E3 ubiquitin ligase complex that mediates the ubiquitination of multiple substrates, leading to either their proteasomal or lysosomal degradation, or modulating their activity through non-degradative ubiquitination (PubMed:36964131, PubMed:35921411). Regulates voltage- gated potassium channels, thereby modulating hyperpolarizing potassium currents in neurons and inducing potassium-dependent membrane hyperpolarization. Also regulates the neuronal glutamine transporter SLC38A2/SAT2, influencing neurotransmitter precursor availability (PubMed:27742667). Regulates calpain activity via atypical, non- degradative ubiquitination by mediating ubiquitination of CAPN1 via 'Lys6'-, 'Lys27'-, 'Lys29'-, and 'Lys63'-linked chains, and CAPN2 via 'Lys6'-linked chains, thereby preventing calpain autolysis (PubMed:36964131). Promotes lysosomal enzyme trafficking and maintains lysosomal function by acting as an endogenous regulator of CLN5, enhancing its ubiquitination and proteasomal degradation (PubMed:35921411). {ECO:0000250|UniProtKB:Q8BJK1, ECO:0000269|PubMed:27742667, ECO:0000269|PubMed:35921411, ECO:0000269|PubMed:36964131}. This is the function of KCTD7 (potassium channel tetramerization domain containing 7, Ensembl gene identifier ENSG00000243335).