Assembly of the Major Light-harvesting Chlorophyll-a/b Complex
The major light-harvesting chlorophyll-a/b complex in most higher plants contains three carotenoids, lutein, neoxanthin, and violaxanthin. How these pigments are assembled into the complex during its biogenesis is largely unknown. Here we show that neoxanthin but not lutein can dissociate from the fully assembled complex. Its equilibrium binding constant in a detergent system (0.1% n-dodecyl-beta-D-maltoside) was determined to be > or = 10(6) m(-1). Neoxanthin insertion into light-harvesting chlorophyll-a/b complex prefolded from overexpressed apoprotein (Lhcb1*2 from Pisum sativum) in the presence of chlorophylls a, b, and lutein as the sole carotenoid is kinetically controlled by an activ…
Structure of Mega-Hemocyanin Reveals Protein Origami in Snails
SummaryMega-hemocyanin is a 13.5 MDa oxygen transporter found in the hemolymph of some snails. Similar to typical gastropod hemocyanins, it is composed of 400 kDa building blocks but has additional 550 kDa subunits. Together, they form a large, completely filled cylinder. The structural basis for this highly complex protein packing is not known so far. Here, we report the electron cryomicroscopy (cryo-EM) structure of mega-hemocyanin complexes from two different snail species. The structures reveal that mega-hemocyanin is composed of flexible building blocks that differ in their conformation, but not in their primary structure. Like a protein origami, these flexible blocks are optimally pac…