Dual Specificity Phosphatases
Dual specificity phosphatases (DSPs) are essential enzymes involved in many aspects of signaling, cell growth and differentiation and the cell cycle. In contrast to conventional tyrosine phosphatases, DSPs display dual phosphatase activity for phospho-tyrosine and phospho- threonine/serine. The first prototype of DSP was identified in the Vaccinia virus late gene product VH1 (Guan et al., Nature, 1991) . To date, VH1 (M.W. ~20kDa) is the prototype of a family of VH1-like DSPs highly abundant in plants, yeast, insects and higher eukaryots .
The human genome encodes at least 38 VH1-like phosphatases, which are involved in the most diverse aspects of the cell cycle.
We recently determined the crystal structure of the Vaccinia virus VH1 at 1.3Å resolution (Koksal et al., J. Biol Chem, 2009). Both in solution and in crystal, VH1 adopts a novel dimeric quaternary structure, stabilized by an N-terminal domain swap (Fig. 1). In dimer of VH1, the two active sites are spaced ~39Å apart from each other.
Figure 1. Dimeric Structure of Vaccinia virus DSP-VH1 determined at 1.3Å resolution (Koksal et al., submitted) (pdb 3CM3). Left panel, ribbon model of the dimeric DSP in two orientations. Right panel, blow-up of the active site visualized at 1.3Å resolution, the highest resolution ever achieved for a dual specificity phosphatase.
Dimeric VH1 is part of in the Vaccinia virion and is released in infected cell upon virus entry. We demonstrated that in the cytoplasm of infected cells, VH1 specifically dephosphorilates the transcription factor STAT1 at Tyr701 and hence blocks its nuclear translocation. Retention of STAT1 in the cytoplasm prevents production of interferon-g to initiate antiviral response. Current work in my laboratory is aimed at better understanding the molecular basis for VH1 dimerization and how this may affects catalysis. We are also interested in studying the more complex dimeric DSP Laforin. Laforin is a human DSP encoded by the EPM2A gene, which is mutated in patients with Lafora disease, a lethal progressive form of epilepsy.