Role of domain swapping in the hetero-oligomeric cytochrome b6f lipoprotein complex.
Domain swapping that contributes to the stability of biologically crucial multisubunit complexes has been implicated in protein oligomerization. In the case of membrane protein assemblies, domain swapping of the iron-sulfur protein (ISP) subunit occurs in the hetero-oligomeric cytochrome b6f and bc1 complexes, which are organized as symmetric dimers that generate the transmembrane proton electrochemical gradient utilized for ATP synthesis. In these complexes, the ISP C-terminal predominantly β-sheet extrinsic domain containing the redox-active [2Fe-2S] cluster resides on the electrochemically positive side of each monomer in the dimeric complex. This domain is bound to the membrane sector of the complex through an N-terminal transmembrane α-helix that is "swapped' to the other monomer of the complex where it spans the complex and the membrane. Detailed analysis of the function and structure of the b6f complex isolated from the cyanobacterium Fremyella diplosiphon SF33 shows that the domain-swapped ISP structure is necessary for function but is not necessarily essential for maintenance of the dimeric structure of the complex. On the basis of crystal structures of the cytochrome complex, the stability of the cytochrome dimer is attributed to specific intermonomer protein-protein and protein-lipid hydrophobic interactions. The geometry of the domain-swapped ISP structure is proposed to be a consequence of the requirement that the anchoring helix of the ISP not perturb the heme organization or quinone channel in the conserved core of each monomer.
†Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907, United States.
Rachna
R
Agarwal
†Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907, United States.
S Saif
SS
Hasan
‡Department of Biology, Indiana University, Bloomington, Indiana 47405, United States.
LaDonna M
LM
Jones
†Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907, United States.
Jason T
JT
Stofleth
§Pasarow Mass Spectrometry Laboratory, NPI-Semel Institute, University of California, Los Angeles, California 90095, United States.
Christopher M
CM
Ryan
§Pasarow Mass Spectrometry Laboratory, NPI-Semel Institute, University of California, Los Angeles, California 90095, United States.
Julian P
JP
Whitelegge
‡Department of Biology, Indiana University, Bloomington, Indiana 47405, United States.
David M
DM
Kehoe
†Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907, United States.
William A
WA
Cramer
D001426:Bacterial Proteins / Q000737:chemistry*; D000458:Cyanobacteria; D045347:Cytochromes b6 / Q000737:chemistry*; D008074:Lipoproteins / Q000737:chemistry*; D008958:Models, Molecular; D054730:Protein Interaction Domains and Motifs; D017433:Protein Structure, Secondary
D001426:Bacterial Proteins; D045347:Cytochromes b6; D008074:Lipoproteins