Identincation of a novel aryl phosphate-binding site in ptp1b: implications for inhibitor design

The structure of the catalytically inactive mutant (C215S) of the human protein-tyrosine phosphatase IB (PTP1B) has been solved to high resolution in two complexes. In the first, crystals were grown in the presence of bis(-4nra-phosphophenyl)methane (BPPM), a synthetic high-affinity low molecular weight non-peptidic substrate (K_m=l6 |iM), and the structure was refined to an R-factor of 18.2% at 1.9 A resolution. In the second, crystals were grown in a saturating concentration of phosphotyrosine (pTyr), and the structure was refined to an R-factor of 18.1 % at 1.85 A. Difference Fourier maps showed that BPPM binds FTP IB in two mutually exclusive modes, one in which it occupies the canonical pTyr-binding site (the active site), and another in which a phosphophenyl moiety interacts with a set of residues not previously observed to bind aryl phosphates. The identification of a second pTyr molecule at the same site in the PTPlB/C215S-pTyr complex confirms that these residues constitute a low-affinity non-catalytic aryl phosphatebinding site. Although the biological relevance of the second aryl phosphate binding site is unclear, its identification provides a new paradigm for the design of tight-binding, highly specific inhibitors for PTP1B.