Tema con variazioni: modulated structures of Hyp-1 complexes
Mariusz Jaskolski(1,2), Joanna Sliwiak(2), Zbigniew Dauter(3)
(1)Department of Crystallography, A. Mickiewicz University; (2)Center for Biocrystallographic Research, Institute of Bioorganic Chemistry, Polish Academy of Science, Poznan, Poland; (3)Synchrotron Radiation Research Section, MCL, National Cancer Institute, Argonne National Laboratory, Argonne, IL, USA;
We reported previously that Hyp-1 (a PR-10 protein from St John’s wort), when cocrystallized with ANS (8-anilino-1-naphthalene sulfonate), formed tetartohedrally twinned crystals with commensurately modulated structure in space group C2, in which seven tetrameric assemblies (each formed from two beta-sheet dimers related by ~180 deg rotation and ~1/14 translation along c) were arranged according to translational non-crystallographic symmetry (tNCS) with nearly perfect repetition of ~1/7 along c. This bizarre packing resulted in a very long c axis (~298 Å), which is also the direction of tetragonal pseudosymmetry. In reciprocal space, there is a salient intensity modulation, with peaks at l=7n+/-3 and troughs in-between. In addition to the unique 28 copies of Hyp-1, the large cell also contains 89 copies of the ANS ligand, 60 of which are intimately bound in a variable pattern in three distinct docking sites of the protein molecules. The unique pattern of ANS docking in the NCS-related Hyp-1 molecules confirms that the modulation is real, and the excellent electron density of the ligands confirms that exact crystallographic periodicity is regained after seven pseudotranslational repetitions along c. There is also an intriguing correlation between the interstitial ANS ligands and a pattern of shorter-longer distances between the tNCS-related protein molecules. Recently, in a different crystallization experiment, which used a mixture of melatonin and ANS as competing ligands, Hyp-1 has been crystallized again producing crystals of the same symmetry and a, b parameters, but with c (384 Å) elongated 9/7 times, and with l=9n+/-4 intensity modulation in the diffraction pattern. The structure has been solved as well, revealing the same packing principle, but with the basic pattern repeated nine times along c. Refinement of this gigantic structure, with 36 unique protein molecules, is a daunting task but it is already obvious that the protein crystallized with selective binding of ANS, and that the same tetrameric NCS assembly is exploited to build a tNCS structure that is a rational (but difficult to explain!) variation of the previous case.