Enzymatic properties of Myxococcus xanthus exopolyphosphatases mxPpx1 and mxPpx2.

Myxococcus xanthus possesses two exopolyphosphatases, mxPpx1 and mxPpx2, which belong to the family of Ppx/GppA phosphatases; however, their catalytic properties have not been described. mxPpx1 and mxPpx2 contain 311 and 505 amino acid residues, respectively; mxPpx2 has an additional C-terminal region, which corresponds to the metal-dependent HDc phosphohydrolase domain. mxPpx1 mainly hydrolyzed short-chain polyPs (polyP and polyP), whereas mxPpx2 preferred long-chain polyP and polyP. mxPpx2 was activated by 25-50 mM KCl, but mxPpx1 did not significantly depend on K. In addition, mxPpx1 and mxPpx2 showed weak hydrolysis of ATP and GTP in the absence of K, and mxPpx2 could also hydrolyze guanosine pentaphosphate (pppGpp) in the presence of K. The exopolyphosphatase activity of mxPpx1 toward polyP was inhibited by polyP and that of mxPpx2 toward polyP and polyP, by pyrophosphate. To clarify the function of the mxPpx2 C-terminal domain, it was fused to mxPpx1 (mxPpx1-2C) and deleted from mxPpx2 (mxPpx2∆C). Compared to wild-type mxPpx2, mxPpx2∆C had significantly reduced exopolyphosphatase activity toward long-chain polyPs (by 90%), whereas that toward polyP and polyP was much less affected; furthermore, the phosphohydrolase activity toward pppGpp, ATP, and GTP was also decreased (by 30-75%). In contrast, mxPpx1-2C had increased hydrolytic activity compared to mxPpx1. Furthermore, mxPpx2∆C lost the requirement for K characteristic for the wild-type enzyme, whereas mxPpx1-2C acquired it. These results suggest that the C-terminal domain of mxPpx2 is necessary for its maximum hydrolytic activity, especially toward long-chain polyPs, and defines mxPpx2 dependency on K for activation.