Regulatory subunit of the IKK complex. Probably recruits IkappaBalpha/NFKBIA to the complex. May be involved in the organization of the cytomatrix at the nerve terminals active zone (CAZ) which regulates neurotransmitter release. May be involved in vesicle trafficking at the CAZ. May be involved in Rab-6 regulated endosomes to Golgi transport. Part of a complex with CHUK, IKBKB and IKBKG. Interacts with CHUK, IKBKB and IKBKG. The interaction with IKBKG is independent of CHUK and IKBKB. Interacts with NFKBIA. Isoform 4 interacts with PPFIA1, and through its C-terminus with the PDZ domains of RIMS1 and RIMS2. Interacts with ERC2/CAST1. Interacts with the GTB-bound forms of RAB6A isoform 1 and isoform 2 and with RAB6B. The interaction was strongest with RAB6B, followed by RAB6A isoform 2 and weakest with RAB6A isoform 1. Widely expressed. Isoform 2 and isoform 4 are abundantly expressed in brain. Isoform 1 and isoform 3 are predominantly expressed in testis and thyroid, and isoform 1 predominates in other tissues tested. 5 isoforms of the human protein are produced by alternative splicing. Note: This description may include information from UniProtKB.