Content area
Full Text
So Iwata,* Joong W. Lee, Kengo Okada,^ John Kyongwon Lee, Momi Iwata, Bjarne Rasmussen, Thomas A. Link, S. Ramaswamy, Bing K. Jap*
Mitochondrial cytochrome bc, complex performs two functions: It is a respiratory multienzyme complex and it recognizes a mitochondrial targeting presequence. Refined crystal structures of the 11-subunit bc, complex from bovine heart reveal full views of this bifunctional enzyme. The "Rieske" iron-sulfur protein subunit shows significant conformational changes in different crystal forms, suggesting a new electron transport mechanism of the enzyme. The mitochondrial targeting presequence of the "Rieske" protein (subunit 9) is lodged between the two "core" subunits at the matrix side of the complex. These "core" subunits are related to the matrix processing peptidase, and the structure unveils how mitochondrial targeting presequences are recognized.
Cytochrome bc^sub 1^ complex (otherwise known as complex III of the respiratory chain or ubiquinol:cytochrome c oxidoreductase) is an oligomeric electron transfer enzyme complex found in the inner mitochondrial membrane of eukaryotes and in the plasma membrane of bacteria (1, 2). This complex is the middle component of the mitochondrial respiratory chain, coupling the transfer of electrons from ubihydroquinone to cytochrome c with the generation of a proton gradient across the mitochondrial membrane. Every bc^sub 1^ complex contains three common subunits with active redox centers cytochrome b, cytochrome c^sub 1^, and the "Rieske" [2Fe-2S] protein (ISP) (3). The mitochondrial system contains additional subunits not present in the bacterial complexes (4, 5).
Bovine heart mitochondrial bc^sub 1^ complex is a dimer, with each monomer consisting of 11 different polypeptide subunits; the primary structures of all 11 subunits are known and the total mass of a monomer is ~240 kD. The "core" subunits (subunits 1 and 2) of the bc^sub 1^ complex have been shown to be members of the mitochondrial processing peptidase (MPP) family of proteins, suggesting that the bcl complex may be a bifunctional protein also involved in mitochondrial import protein processing (6, 7).
There have been recent advances toward the goal of a complete atomic structure of the bcl complex. Deisenhofer and Yu's group reported a partial structure of the bc^sub 1^ complex from bovine heart mitochondria based on their tetragonal space group crystals (8). In their structure, 5 of the 11 subunits are completely assigned...