March 9th, 2006
Symmetry is an important concept in both biology and physics. This is especially apparent in the field of membrane biochemistry, where the recently-solved 3-dimensional structures of bioenergetic complexes occur in the biological membrane as a dimer or a trimer of the asymmetric unit with a corresponding C2 or C3 axis of symmetry. Photosystem I is particularly interesting because the reaction center core is a pseudo-C2-symmetric heterodimer that likely evolved from a C2-symmetric homodimeric precursor. This change was accompanied by the recruitment of a small, bacterial dicluster ferredoxin, now known as PsaC, which serves as the terminal electron acceptor. The involvement of FA and FB in electron transfer lengthened the time of charge separation, thereby ensuring a high quantum yield. The FX region on the PsaA/PsaB heterodimer is highly symmetric, yet PsaC binds asymmetrically (i.e. in one of two possible orientations). For PsaC to bind in a single orientation, a number of minor alterations were necessary in the structures of PsaC and the PsaA/PsaB heterodimer. We have studied the assembly of these subunits by comparing the three-dimensional NMR solution structure of unbound PsaC (1K0T) with the atomic-resolution X-ray crystal structure of PsaC bound to the Photosystem I reaction center (1JB0). This is the first instance in which bound and unbound three-dimensional structures are available for a membrane-associated protein. We found structural features, specific to unbound PsaC in solution, which prevent binding of this protein to the Photosystem I core in the incorrect orientation. Hence, the information necessary to bind correctly is coded in the primary amino acid sequence of these proteins. We show that in its approach to the Photosystem I core, PsaC undergoes a step-wise structural change that assures its binding in the correct orientation. Similar fundamental principles may well have evolved in the assembly of membrane-associated subunits of other symmetrical membrane complexes.
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