Component of the cullin-RING ubiquitin ligases (CRL), or CUL3-RBX1-BTB protein E3 ligase complexes which mediate the ubiquitination and subsequent proteasomal degradation of target proteins. The functional specificity of the CRL complex depends on the BTB domain-containing protein as the susbstrate recognition component. Involved in embryo pattern formation and endosperm development. Required for the normal division and organization of the root stem cells and columella root cap cells. Regulates primary root growth by an unknown pathway, but in an ethylene-dependent manner. Functions in distal root patterning, by an ethylene-independent mechanism. Functionally redundant with CUL3A. Protein modification; protein ubiquitination. Interacts with BTB/POZ-MATH proteins BPM1 and BPM3. Expressed in developing and mature reproductive organs and during embryogenesis. Neddylated. Deneddylated via its interaction with the COP9 signalosome (CSN) complex. No visible phenotype. Cul3a and cul3b double mutant is embryonic lethal (PubMed:16045478). Belongs to the cullin family.