Molecular Biology Laboratory, Institute for Chemical Research (ICR), Kyoto University

The tale of the 3’ UTR tail: shedding light on how CSN regulates signal transduction in plants and mammals ~ can it bridge proteolysis to pre-mRNA processing?

COP9 signalosome (CSN) is a 8-subunit nuclear protein complex, highly conserved among mammals to plants. CSN is a key hub for signal transduction initiated from stimuli such as such as light, UV, DNA damage, starvation, oncogenesis, etc. To date, CSN function has been mostly attributed to proteolysis-mediated gene expression regulation through CSN5 subunit. In addition, we have discovered an intrinsic function of gene expression repression through CSN1 subunit that is independent of proteolysis.

Interestingly, pre-mRNA processing proteins from the spliceosome and 3' UTR processing machineries were found to bind CSN1 at its N-terminus in mammals and plants. In order to comprehend this novel regulation of gene expression through CSN1, we have been characterizing the molecular mechanism of these RNA processing proteins, in a model plant.

In Arabidopsis thaliana, when the gene function of the CSN-binding spliceosomal protein, SAP130, was knocked-down, pollen development was impaired during the transition from microspore to bicellular stages. Interestingly, analyses on partially complemented plants with reduced CSN1 function also showed pollen developmental defects, identical to those seen in AtSAP130 knockdown plants. Taken together, we hypothesize CSN1 and SAP130 coordinately regulate pollen development, in a spatiotemporal manner.

Furthermore, CSN1 has been shown to bind CFI 68, a member of the Cleavage Factor I (CFI) protein complex, responsible for alternative polyadenylation regulation at the 3' ends of pre-mRNAs. Through detailed analyses, here we show novel CFI functions, by taking advantage of the plant system.

First, AtCFIfunction was shown to be essential for not only maintaining the length but also the diversity of 3’ UTR polyadenylation sites, while some the CFI subunits shared redundant function in this molecular function. Second, genome-wide analyses showed that genes involved in light signaling and photosynthesis are significantly dependent on AtCFI function. Third, in silico structure modeling demonstrated that the N-terminal portion of CSN1 docks onto the surface formed between CFI 25 and CFI 68 on the CFI. Interestingly, this N-terminal portion of CSN1 was also shown to swing out towards SAP130, the spliceosome subunit, to dock onto its protruding helical insertion structure. These models suggest a novel mode of interaction among mega protein complexes.