PARENT SESSION
Posters P7B Evolution of photosynthesis. Abstracts (579-591)

Breaking biological symmetry in membrane proteins: How PsaC evolved to orient asymmetrically on the Photosystem I core. John Golbeck^*,, Mikhail Antonkine, Dietmar Stehlik,

ABSTRACT- Symmetry is as important in biology as in physics. This is especially true in the field of membrane biochemistry, where the recently-solved 3-dimensional structures of bioenergetic complexes such as bovine heart cytochrome oxidase (1V54), the mitochondrial cytochrome bc₁ complex (1NTM), the chloroplast cytochrome b₆f complex (1UM3 and 1Q90), cyanobacterial PS II (1FE1 and 1IZL), and cyanobacterial PS I (1JB0) occur in the biological membrane as a dimer or a trimer of the asymmetric unit with a corresponding C₂ or C₃ axis of symmetry. PS I is particularly intriguing 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 bacterial dicluster ferredoxin, now known as PsaC, which serves as the terminal electron acceptor. The involvement of F_A and F_B in electron transfer lengthened the time of charge separation, thereby ensuring a high quantum yield. The F_X region on PsaA/PsaB is highly symmetric, yet PsaC binds asymmetrically (i.e. in one of two possible orientations). For PsaC to bind asymmetrically, a number of alterations were necessary in the structures of both PsaC and the PsaA/PsaB heterodimer. We have studied the assembly of these subunits by comparing the 3-dimensional NMR solution structure of unbound PsaC (1K0T) with the atomic-resolution X-ray crystal structure of PsaC bound to the PS I reaction center. This is the first instance in which bound and unbound atomic-level structures are available for a membrane-associated protein. There exists a simple and elegant set of movements in the 3-dimensional structure of PsaC that prevent it from binding to the PS I core in the incorrect orientation, and assure its binding in the correct orientation. Similar fundamental principles may have evolved in the assembly of membrane-associated subunits of other symmetrical bioenergetic complexes. Supported by the NSF (to JHG).

KEY WORDS: PsaC, Evolution, Iron-sulfur cluster, Assembly