A quest for the atomic resolution of plant photosystem I

Plant photosystem I (PSI) is one of the most intricate membrane complexes in nature. It comprises two distinct building blocks; the reaction center and the lightharvesting complexes (LHC). They contain 16 subunits and over 200 prosthetic groups, including chlorophylls, carotenoids, quinines, phospholipids, glycolipids, and three iron-sulfur clusters. Our group was the first to determine the crystal structure of plant PSI. That first structure was solved to 4.4 Å resolution. Over the last 14 years, we have constantly improved the purification procedure, crystallization conditions, and phasing, by combining numerous datasets. Currently, we have approached 2 Å resolution, which well defines the presence of chlorophylls a and b and the various carotenoids. The crystal structure of plant PSI was solved from two distinct crystal forms. The first was crystallized at pH 6.5 and exhibited P21 symmetry. The second was crystallized at pH 8.5 and exhibited P212121 symmetry. The surfaces involved in binding plastocyanin and ferredoxin were identical in both crystal forms.