A structural investigation of complex I and I+III2 supercomplex from Zea mays at 11-13 angstrom resolution: Assignment of the carbonic anhydrase domain and evidence for structural heterogeneity within complex I

The projection structures of complex I and the I+III2 supercomplex from the C-4 plant Zea mays were determined by electron microscopy and single particle image analysis to a resolution of up to 11 angstrom. Maize complex I has a typical L-shape. Additionally, it has a large hydrophilic, extra-domain attached to the centre of the membrane arm on its matrix-exposed side, which previously was described for Arabidopsis and which was reported to include carbonic anhydrase subunits. A comparison with the X-ray structure of homotrimeric gamma-carbonic anhydrase from the archaebacterium Methanosarcina thermophila indicates that this domain is also composed of a trimer. Mass spectrometry analyses allowed to identify two different carbonic anhydrase isoforms, suggesting that the gamma-carbonic anhydrase domain of maize complex I most likely is a heterotrimer. Statistical analysis indicates that the maize complex I structure is heterogeneous: a less-abundant type II particle has a 15 angstrom shorter membrane arm and an additional small protrusion on the intermembrane-side of the membrane arm if compared to the more abundant type P particle. The I+III2 supercomplex was found to be a rigid structure which did riot break down into subcomplexes at the interface between the hydrophilic and the hydrophobic arms of complex I. The complex I moiety of the supercomplex appears to be only of type I. This would mean that the type II particles are not involved in the supercomplex formation and, hence, could have a different physiological role. (C) 2007 Elsevier B.V. All rights reserved.