An electron microscopic study of the archaeal feast/famine regulatory protein: 2. Crystal formation by a full length FFRP, pot0434017 (FL11)

In the crystal the archaeal feast/famine regulatory protein pot0434017 (FL11) forms helical cylinders, each extending along the c axis and densely packed with a hexagonal symmetry in the a/b plane. By applying cryo-electron microscopy to protein solutions yielding crystals, with selectively focussing zero-loss electrons accelerated at 200 KV, two types of regular objects were observed, hexagonal (i.e. projections of crystals to their a/b planes) or rectangular (i.e. projections onto planes perpendicular to a/b). The two types of images are different in the ranges of sizes, suggesting that the crystallization might initiate by forming a hexagonal sheet on the a/b plane, subsequently extending along the c axis. Some other images obtained were intermediate between regular and amorphous, suggesting that some crystals were growing inside amorphous precipitates by rearranging the protein molecules, and that some larger crystals were growing by absorbing smaller amorphous precipitates. Tubes running parallel to each other were also observed in pieces of thin films. These tubes have hollows in their centers, and their lateral arrangement with a periodicity of ~150 Å and the presence of a helical component ~50 Å suggest that they are projections of the helical cylinders, forming mono-layers on the a/c or b/c planes.