AURACYANIN B STRUCTURE IN SPACE GROUP P6₅

Mihwa Lee,^a Megan J. Maher,^a Robert E. Blankenship,^b Hans C. Freeman,^a and

J. Mitchell Guss ^a

^aSchool of Molecular and Microbial Biosciences, University of Sydney, NSW 2006, Australia; ^bDepartment of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287-1604, USA (m.lee@mmb.usyd.edu.au)

Auracyanin B is one of two similar blue copper proteins produced by the thermophilic green photosynthetic bacterium Chloroflexus aurantiacus [1]. It is likely to be involved in photosynthetic electron transfer in this organism. The structure of auracyanin B has previously been solved and refined in the hexagonal space group P6₄22 with a single molecule in the asymmetric unit (unit cell parameters a = b = 115.7, c = 54.5 Å) [2].

The protein has now been crystallized in a new crystal form, P6₅ with unit cell parameters, a = b = 115.9, c = 108.2 Å. In the new crystal form the asymmetric unit contains four protein molecules. The structure has been solved by molecular replacement and refined at 1.9 Å resolution. In relation to the earlier crystal structure, the c-axis of unit cell is doubled and the number of molecules is increased from one to four. The main-chain structure of auracyanin B in P6₅ is very similar to that in P6₄22 but some residues including those involved in the crystal contacts have slightly different side chain conformations. The other interesting feature of this structure in P6₅ is that the operation which relates two pairs of molecules in the asymmetric unit is very close to a formal crystallographic two-fold axis, which could yield space group P6₅22. Yet the structure could be refined only in the lower symmetry space group P6_5. The final residuals are R = 19.2% and R_free = 22.0%.

References

1 McManus, J. D., Brune, D. C., Han, J., Sanders-Loehr, J., Meyer, T. E., Cusanovich, M. A., Tollin, G., and Blankenship, R. E. (1992) J. Biol. Chem. 267, 6531-40.

2 Bond, C. S., Blankenship, R. E., Freeman, H. C., Guss, J. M., Maher, M. J., Selvaraj, F. M., Wilce, M. C. J., and Willingham, K. M. (2001) J. Mol. Biol. 306, 47-67.