Nucleic Acids Research Advance Access published online on April 29, 2008
Nucleic Acids Research, doi:10.1093/nar/gkn079
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Structural Biology |
Crystal structure of the 25 kDa subunit of human cleavage factor Im
1Department of Microbiology and Department of Molecular Genetics, University of Vermont, Burlington, VT 05405, USA, 2Department of Cell Biology, Biozentrum, University of Basel, CH-4056 Basel, Switzerland and 3Molecular Physiology and Biophysics, University of Vermont, Burlington, VT 05405, USA
*To whom correspondence should be addressed. Tel: +1 802 656 9531; Fax: +1 802 656 8749; Email: sdoublie{at}uvm.edu
Received November 2, 2007. Revised February 8, 2008. Accepted February 9, 2008.
Cleavage factor Im is an essential component of the pre-messenger RNA 3'-end processing machinery in higher eukaryotes, participating in both the polyadenylation and cleavage steps. Cleavage factor Im is an oligomer composed of a small 25 kDa subunit (CF Im25) and a variable larger subunit of either 59, 68 or 72 kDa. The small subunit also interacts with RNA, poly(A) polymerase, and the nuclear poly(A)-binding protein. These protein–protein interactions are thought to be facilitated by the Nudix domain of CF Im25, a hydrolase motif with a characteristic 
/β/ fold and a conserved catalytic sequence or Nudix box. We present here the crystal structures of human CF Im25 in its free and diadenosine tetraphosphate (Ap4A) bound forms at 1.85 and 1.80 Å, respectively. CF Im25 crystallizes as a dimer and presents the classical Nudix fold. Results from crystallographic and biochemical experiments suggest that CF Im25 makes use of its Nudix fold to bind but not hydrolyze ATP and Ap4A. The complex and apo protein structures provide insight into the active oligomeric state of CF Im and suggest a possible role of nucleotide binding in either the polyadenylation and/or cleavage steps of pre-messenger RNA 3'-end processing.