Structure of Thermus thermophilus HB8 H-protein of the glycine-cleavage system, resolved by a six-­dimensional molecular-replacement method

The glycine-cleavage system is a multi-enzyme complex consisting of four different components (the P-, H-, T- and L-proteins). Recombinant H-protein corresponding to that from Thermus thermophilus HB8 has been overexpressed, purified and crystallized. Synchrotron radiation from BL44B2 at SPring-8 was used to collect a native data set to 2.5 Å resolution. The crystals belonged to the hexagonal space group P6₅ and contained three molecules per asymmetric unit, with a solvent content of 39%. Because of the large number of molecules within a closely packed unit cell, this structure was solved by six-dimensional molecular replacement with the program EPMR using the pea H-protein structure as a search model and was refined to an R factor of 0.189 and a free R factor of 0.256. Comparison with the pea H-protein reveals two highly conserved regions surrounding the lipoyl-lysine arm. Both of these regions are negatively charged and each has additional properties that are conserved in H-proteins from many species, suggesting that these regions are involved in intermolecular interactions. One region has previously been proposed to constitute an interaction surface with T-protein, while the other may be involved in an interaction with P-­protein. Meanwhile, the lipoyl-lysine arm of the T. thermophilus H-protein was found to be more flexible than that of the pea H-protein, supporting the hypothesis that H-­protein does not form a stable complex with L-protein during the reaction.