Substrate-specific adapter of a BCR (BTB-CUL3-RBX1) E3 ubiquitin ligase complex that acts as a multifunctional regulator of B- cell receptor (BCR) signaling, germinal center formation, and T-cell function. Functions by mediating polyubiquitination of target proteins, leading to their proteasomal degradation. In B-cells, plays an essential role in BCR signaling and germinal center B-cell maturation. Targets the BCR signaling subunits CD79A and CD79B for ubiquitin- mediated degradation, thereby controlling surface BCR homeostasis (PubMed:38630892). Mediates polyubiquitination and degradation of the mRNA decay factor roquin-2/RC3H2, which in turn allows accumulation of TNFAIP3, a negative regulator of NF-kappa-B signaling, thereby providing feedback inhibition of BCR-induced NF-kappa-B activation (PubMed:29695787). In T-cells, acts as a dual negative regulator of T- cell exhaustion and mitochondrial dysfunction. Promotes 'Lys-48'-linked polyubiquitination and proteasomal degradation of the transcription factor TOX, thereby attenuating the transition of progenitor exhausted T-cells towards terminal exhaustion (PubMed:41535474). Also targets the mitochondrial phosphatase PGAM5 for degradation, thereby preventing excessive PGAM5-mediated dephosphorylation and activation of DRP1, which would otherwise drive pathological mitochondrial fission (PubMed:41535474). Through these mechanisms, maintains mitochondrial fitness in T-cells during chronic antigen stimulation. {ECO:0000269|PubMed:29695787, ECO:0000269|PubMed:38630892, ECO:0000269|PubMed:41535474}. This is the function of ENSG00000172578 (KLHL6, kelch like family member 6).