Three-dimensional organization of retroviral capsid proteins on a lipid monolayer

We have used a method for the two-dimensional crystallization of retro-viral structural proteins to obtain a three-dimensional structure of negatively stained, membrane-bound, histidine-tagged Moloney murine leukemia virus (M-MuLV) capsid protein (his-MoCA) arrays. Tilted and untilted micrographs from crystals formed by purified his-MoCA proteins incubated beneath lipid monolayers containing nickel-chelating lipids were used in 3D reconstructions. The 2D crystals had unit cell dimensions of a = 72.6 Å, b = 72.5 Å and γ = 119.5°, but appeared to have no intrinsic symmetry (p1) in 3D, in contrast to the trigonal or hexagonal appearance of their 2D projections. Membrane-bound his-MoCA proteins showed a strand-like organization, apparently with dimer building blocks, Membrane-proximal regions, or putative N-terminal domains (NTDs), dimerized with different partners than the membrane-distal putative C-terminal domains (CTDs). Evidence also suggests that CTDs can adopt alternate orientations relative to their NTDs, forming inter-strand connections, Our results are consistent with helical-spiral models for retrovirus particle assembly, but are not easily reconcilable with icosa- hedral models. (C) 2000 Academic Press.