Planar-tubular two-dimensional (2D) crystals of human vitamin K-dependenty gamma-glutamyl carboxylase grow in the presence of dimyristoyl phosphatidylcholine (DMPC). Surprisingly, these crystals form below the phase transition temperature of DMPC and at the unusually low molar lipid-to-protein (LPR) ratio of 1, while 2D crystals are conventionally grown above the phase transition temperature of the reconstituting lipid and significantly higher LPRs. The crystals are up to 0.75 mu m in the shorter dimension of the planar tubes and at least I pm in length. Due to the planar-tubular nature of the crystals, two lattices are present. These are rotated by nearly 90 degrees in respect to each other. The ordered arrays exhibit p12(1) plane group symmetry with unit cell dimensions of a = 83.7 angstrom, b = 76.6 angstrom, gamma = 91 degrees. Projection maps calculated from images of negatively stained and electron cryo-microscopy samples reveal the human vitamin K-dependent gamma-glutamyl carboxylase to be a monomer. (c) 2006 Elsevier Inc. All rights reserved.