Structure-Function of Two Membrane Protein Systems Involved in Transfer of Electrical Charge

Abstract

Structure-function relationships are presented for two membrane proteins or protein complexes that are involved in the transfer of electrical charge through or across biological membranes: (i) the cytochrome b ₆ f complex and its subunit, cytochrome f, that transfer electrons on a sub-millisecond time scale from the lipophilic quinol to the photosystem I reaction center in oxygenic photosynthetic membranes. Cytochrome f carries out one step in this transfer by oxidizing the membrane-bound iron-sulfur [2Fe-2S] protein in the complex and reducing its soluble electron acceptor, plastocyanin. (ii) Colicin El is a toxin-like bactericidal protein that forms a voltage-gated ion channel with a single channel conductance of 10⁶–10⁷ ions/channel/sec in the E. coli cytoplasmic membrane.

The structure of the active lumen-side domain (252 residues) of cytochrome f has been solved at atomic (1.96Å) resolution, the first subunit of the integral polytopic cytochrome bc ₁ or b ₆ f complexes for which an atomic structure has been obtained. The existence of two domains, a predominantly β-strand motif, and the use of the N-terminal amino group as the sixth ligand are unique features of the structure.

The colicin ion channel contrasts with the cytochrome complex in not being permanently embedded in the membrane, but in making a transition, involving a very large structural change, from a water-soluble to an integral membrane-bound state. This kind of structural change is characteristic of the mode of action of many toxins, such as diphtheria toxin, and toxin-like molecules such as the colicins. The voltage-driven gating of the channel, in which the colicin channel is converted from a closed to an open ion-conductive state, involves import into the membrane of at least one trans-membrane amphiphilic helical hairpin.

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