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Structure of the membrane-pore-forming fragment of colicin A
M W Parker1, F Pattus, A D Tucker
1European Molecular Biology Laboratory, Heidelberg, FRG.
Abstract:
Colicins are antibiotic proteins produced by and active against sensitive Escherichia coli and closely related bacteria. They can adsorb to specific receptors located at the external surface of the outer membrane of sensitive cells, and are then translocated to their specific targets within these cells. The largest group of colicins comprises those which can form voltage-dependent channels in membranes, thereby destroying the cell's energy potential. Colicin molecules are organized in structural domains, each domain carrying one function associated with the toxin's lethal activity. The pore-forming activity seems to be located at the carboxyl terminus. A thermolytic fragment comprising amino acids 389-592 from colicin A has pore-forming properties very similar to those of the entire molecule. This fragment is soluble in aqueous medium and spontaneously inserts into lipid bilayers. We have determined the structure of the pore-forming fragment of colicin A by X-ray crystallography and refinement at 2.5 A resolution. The protein consists of ten alpha-helices organized in a three-layer structure. Two of the helices are completely buried within the structure and form a hydrophobic hairpin loop similar to that proposed for signal sequences which function in translocation. We present a model for insertion of the protein into lipid bilayers the features of which may be applicable in other biological systems involving protein insertion or translocation across membranes.
Insights
Colicin A, a pore-forming protein, was structurally analyzed using X-ray crystallography. Its pore-forming fragment reveals a ten alpha-helix structure, offering insights into membrane insertion and translocation mechanisms.
Area of Science:
- Biochemistry
- Structural Biology
- Microbiology
Background:
- Colicins are bacteriocins produced by E. coli, acting against sensitive strains.
- Pore-forming colicins disrupt bacterial membranes by creating voltage-dependent channels.
- Colicin A's pore-forming domain is located at its carboxyl terminus.
Purpose of the Study:
- To determine the 3D structure of the pore-forming fragment of colicin A.
- To elucidate the structural basis of colicin A's membrane insertion and channel formation.
- To propose a model for protein translocation across lipid bilayers.
Main Methods:
- X-ray crystallography at 2.5 A resolution.
- Protein fragment purification and characterization.
- Structural refinement and analysis.
Main Results:
- The pore-forming fragment of colicin A comprises ten alpha-helices arranged in a three-layer structure.
- Two alpha-helices form a buried hydrophobic hairpin loop, potentially involved in membrane insertion.
- The structure provides a basis for understanding spontaneous insertion into lipid bilayers.
Conclusions:
- The determined structure of colicin A's pore-forming fragment provides atomic-level detail of its membrane-interacting domain.
- The hydrophobic hairpin loop is a key feature for membrane penetration and translocation.
- This structural information can inform models of protein insertion and translocation in biological membranes.