What better way to cut short this unanticipated hiatus (no real reason – just got busy with other stuff) than point readers to a new study that reveals yet another unexpected and unusual property of a plant polyadenylation factor subunit. In a nutshell (which is posted, as the abstract, beneath the fold), Balu Addepalli has found that two of the cysteines in one of the three CCCH zinc finger motifs in the Arabidopsis CPSF30 protein (that’s a mouthful) are actually engaged in a disulfide bond. This was unexpected (to me, at least) because it is usually assumed that the cysteines and histidine residue coordinate around a zinc ion (hence the term zinc finger). As indicated in the abstract, this new finding raises the possibility that the Arabidopsis CPSF30 is regulated in part by redox control, through oxidation and reduction of the disulfide bond.
So, this adds one more item to the cart of novelties regarding the plant CPSF30. The gene encoding the protein is alternatively spliced, such that the CPSF30 part of the resulting protein is fused to a novel, poorly-understood domain. (See this article for the details.) The protein is not essential for growth (in contrast to its yeast counterpart, Yth1p). CPSF30 mutants (we call them oxt6 mutants) are tolerant to oxidative stress. (This may be relevant to the disulfide linkage story – future research will tell). The Arabidopsis CPSF30 is an endonuclease, an activity for which a function has not been firmly established (although my group thinks we know – see this study). This protein is also a calmodulin-binding protein, which raises interesting possibilities concerning regulation, signaling, and polyadenylation.
The Arabidopsis ortholog of the 30 kDa subunit of the cleavage and polyadenylation factor (AtCPSF30) is an RNA binding endonuclease, and the endonuclease activity is inhibited by reducing agents. Here, we report the presence of a disulfide linkage in the endonuclease motif based on comparative mass spectrometry (MS) analysis of reduced and non-reduced but carbamidomethylated protein. This analysis reveals that this disulfide bond involves a CCCH zinc finger motif, one that is associated with the endonuclease activity of AtCPSF30. This finding raises the possibility that redox regulation of AtCPSF30 may occur through oxidation and reduction of the disulfide linkage.
The citation (and link, again):
A postscript – I like this paper because it is the fruit of a collaboration with Balu and Pat Limbach (both at the University of Cincinnati) that has brought protein mass spectroscopy to my research. We have a few more irons on the fire, and I am hopeful that some additional structural studies will add even more to our understanding (or confusion, depending on your perspective) of the plant polyadenylation complex.