We have used an in vitro system that mimics the assembly of immature Moloney murine leukemia virus (M-MuLV) particles to examine how viral structural (Gag) proteins oligomerize at membrane interfaces. Ordered arrays of histidine-tagged Moloney capsid protein (his-MoCA) were obtained on membrane bilayers composed of phosphatidylcholine (PC) and the nickel- chelating lipid 1,2-di-O-hexadecyl-sn-glycero-3-(1'-2'-R-hydroxy-3'N-(5- amino-1-carboxypentyl)iminodiacetic acid)propyl ether (DHGN). The membrane- bound arrays were analyzed by electron microscopy (EM) and atomic force microscopy (AFM). Two-dimensional projection images obtained by EM showed that bilayer-bound his-MoCA proteins formed cages surrounding different types of protein-free cage holes with similar cage holes spaced at 81.5-Å distances and distances between dissimilar cage holes of 45.5 Å. AFM images, showing topological features viewed near the membrane-proximal domain of the his-MoCA protein, revealed a cage network of only symmetrical hexamers spaced at 79-Å distances. These results are consistent with a model in which dimers constitute structural building blocks and where membrane-proximal and distal his-MoCA regions interact with different partners in membrane-bound arrays.
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