Plant COP9 signalosome subunit 5, CSN5.

CSN5 is a subunit of the COP9 signalosome (CSN) and carries the metallo-protease catalytic center for the complex. This highly conserved gene has been a subject of intense research in part because human Csn5 (Jab1) has been tightly linked to cancer. We briefly summarize recent research advances on the structure and mechanisms of the CSN in general, and then focus on the Arabidopsis CSN5 genes and their products, AtCSN5A and AtCSN5B. We also briefly discuss CSN6 genes, which are closely related share many similarities to CSN5. CSN5 and CSN6 genes are duplicated in mustard family of plants as well as in several plant species that have no phylogenetic correlation. Sequence homology comparison further suggests that at least some of the duplication events occurred independently. We review and analyze the phenotypic and expression differences of the two CSN5 genes in Arabidopsis, and suggest that they play overlapping as well as specialized roles in plant development. Arabidopsis CSN5 protein sequences are more similar to those of complex organisms such as humans than to yeasts and unicellular alga, suggesting that the structure and mechanism of Arabidopsis CSN5 likely resembles more to those of human than to yeast. We argue that possession of two different isoforms of CSN5s gives Arabidopsis a unique advantage as a genetic model of CSN5 to dissect the multifaceted functions and mechanistic versatilities of this important cellular regulator.