CC2D1A was first identified by Matsuda et al.
) as an NF-κB activator through a large scale screen of human genes. Although CC2D1A was annotated as a mitochondrial protein, our analyses using subcellular fractionation and immunofluorescent microscopy indicate that it is predominantly in the cytoplasm (data not shown). Nevertheless, CC2D1A is one of the most potent activators of NF-κB when it is overexpressed in cells. Furthermore, it has unique DM14 and C2 domains that have never been observed in other NF-κB activators. It is therefore interesting to understand how CC2D1A activates NF-κB.
In this study, we show that CC2D1A activates NF-κB through a canonical signaling cascade involving TRAF2, Ubc13, TAK1, and IKK. The involvement of Ubc13 in this pathway suggests that Lys-63 polyubiquitination is involved in IKK activation by CC2D1A. This is further supported by our data that the Lys-63-specific deubiquitinase CYLD inhibits NF-κB activation by CC2D1A.
NF-κB is activated by a large variety of agents in different cells types. An important question that remains to be resolved is the specific signaling pathway in which CC2D1A plays a role. We have examined the potential role of CC2D1A in NF-κB activation by different agents, including TNFα, interleukin-1β, conditions that cause endoplasmic reticulum stress, as well as various agonists of Toll-like receptors, epidermal growth factor receptor and T cell receptor. So far, we have no evidence in support of a role of CC2D1A in signaling by any of these agents. Given the unusual domain structure of CC2D1A, it is possible that this protein is involved in a novel pathway of NF-κB activation. It also is possible that CC2D1A plays a role in NF-κB activation in specific cell types (e.g. neuronal cells; see below).
Mutations in the gene encoding CC2D1A have been associated with nonsyndromic mental retardation in human (23
). The mutation is a deletion of 3567 nucleotides from introns 13 to16, abolishing the fourth DM14 domain and C2 domain. We have tested a similar mutation in NF-κB luciferase assay (ΔC4; A
). It retains ~20% activity of the full-length protein, suggesting that this is a hypomorphic mutation. NF-κB activity is prominent in the central nervous system throughout development. Several brain-specific factors such as the neurotransmitter glutamate and neurotrophins (24
) have been shown to activate NF-κB. Genetic evidence indicates that NF-κB is involved in learning and memory (25
). Future studies should determine if and how CC2D1A regulates NF-κB in the brain.
It is likely that the functions of CC2D1A are not limited to NF-κB activation. Indeed, the Drosophila
ortholog of CC2D1A, known as Lgd (Lethal (2
) giant discs), is a negative regulator of Notch signaling and involved in endosomal trafficking (18
). Further studies of CC2D1A in an animal model should help delineate the physiological functions of this unusual protein and provide insights into human diseases such as mental retardation.