Structure of the lamprey yolk lipid-protein complex lipovitellin-phosvitin at 2.8 A resolution.

The X-ray crystallographic structure of the lipid-protein complex lipovitellin-phosvitin has been determined with the multiple isomorphous replacement method using four heavy-atom derivatives. Lamprey yolk lipovitellin-phosvitin is a dimeric molecule of molecular weight 352,000. The monomer consists of three polypeptide chains. The smallest is known as phosvitin and has an extremely high phosphoserine content. The monomeric unit also contains about 16% (w/w) of non-covalently bound lipid, probably in a monolayer or bilayer-like configuration. Within each monomer is a "cavity" or region of low electron density. The cavity has a volume of about 68,000 A3 and is believed to contain the lipid in a presumably disordered state. The cavity is roughly conical in shape and is lined on two sides by seven and eight-stranded antiparallel beta-sheets. The base of the cavity opens away from the intersubunit interface, but appears partially closed off from solvent regions by additional antiparallel beta-sheet structure. The beta-sheets lining the sides of the cavity are surrounded by a shell of two curved layers of 16 interconnected helices. The helices in either layer of the shell are all roughly parallel to each other and antiparallel to all of the helices of the other layer. The connectivity of the helices resembles a "superhelix" and is different from the connectivities seen in proteins containing four-helix bundles. There are an estimated 1300 amino acids in lamprey lipovitellin-phosvitin and almost 1000 alanine residues have been modeled into electron density. The remaining residues are assumed to be disordered.