Ssu72 Discussion

Function

In attempting to infer the function of the Drosophila Ssu72 protein, two broad sources of information were used, each leading to competing conclusions which may or may not be incompatible. The first, which is predicated on the broad similarity in amino acid sequence that was found to exist between this protein and the human Ssu72 protein, is in vivo and in vitro experimental evidence about the latter protein, which tends to suggest that the protein in Drosophila also functions as a serine phosphatase, presumably at the C-terminal domain of RNA polymerase II (Krishnamurthy et al., 2004), and has a role in 3' end mRNA processing that is independent of its activity as a phosphatase. The second piece of guidance comes from the results generated by the use of computational and bioinformatic tools in this paper, which prima facie reveal the Drosophila protein to be a protein tyrosine phosphatase. Therefore, these two seemingly conflicting conclusions must now be evaluated on the basis of the limited evidence available.

Protein Tyrosine Phosphatase?

An evaluation of the Drosophila protein as a potential Protein Tyrosine Phosphatase (PTP) is a logical place to begin this discussion as this is what Meinhart, Silberzahn, & Cramer (2003) concluded about the human protein, based on a characterization of its primary, secondary and 3D structures, as well as experimental data. They began by identifying, within the amino acid sequence of the Ssu72 gene, the PTPase signature motiff at the N-terminal end. This is a short sequence, the consensus sequence being (H/V)C(X)5R(S/T), which forms the phosphate-binding site (P-loop) in the mature protein and is where, invariably, members of the PTP superfamily exert their catalytic action. According to ... ( ), 'amino acid sequence comparisons of the catalytic domains of PTPases with the catalytic subunits of protein Ser/Thr phosphatases have shown no sequence similarity'. This does necessarily mean that

Structure

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Abstract | Introduction | Results | Discussion | Method | References