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Publication Abstract from PubMed

Reversible phosphorylation of the C-terminal domain (CTD) of eukaryotic RNA polymerase II largest subunit represents a critical regulatory mechanism during the transcription cycle and mRNA processing. Ssu72 is an essential phosphatase conserved in eukaryotes that dephosphorylates phosphorylated Ser5 (phos.Ser5) of the CTD heptapeptide. Its function is implicated in transcription initiation, elongation and termination as well as RNA processing. Here we report the high resolution x-ray crystal structures of Drosophila melanogaster Ssu72 phosphatase in the apo form and in complex with an inhibitor mimicking the transition state of phosphoryl transfer. Ssu72 facilitates dephosphorylation of the substrate through a phosphoryl-enzyme intermediate, as visualized in the complex structure of Ssu72 with the oxoanion compound inhibitor vanadate at 2.35 A resolution. The structure resembles the transition state of the phosphoryl transfer with vanadate exhibiting a trigonal bi-pyramidal geometry covalently bonded to the nucleophilic cysteine. Interestingly, the incorporation of oxoanion compounds greatly stabilizes a flexible loop containing the general acid as detected by an increase of melting temperature of Ssu72 detected by differential scanning fluorimetry. Ssu72 structure exhibits a core fold with a similar topology to that of low molecular weight protein tyrosine phosphatases (LMWPTP), but with an insertion of a unique "cap" domain to shelter the active site from the solvent with a deep groove in between where the CTD substrates bind. Mutagenesis studies in this groove established the functional roles of five residues (Met17, Pro46, Asp51, Tyr77 and Met85) that are essential specifically for substrate recognition.

Crystal structure of Ssu72, an essential eukaryotic phosphatase specific for the C-terminal domain of RNA polymerase II, in complex with a transition state analogue.,Zhang Y, Zhang M, Zhang Y Biochem J. 2011 Jan 5. PMID:21204787^[1]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.