Orf and vaccinia virus preparations adsorbed to carbon coated grids were treated in various ways. Two general patterns of degradation were noted. The particle lost its shape and increased in surface area when the internal envelope disintegrated. When the membranous external envelope was degraded no change in virus size or shape was noted. The range of agents that degraded the external envelope suggests that it is a lipid-protein (probably proteolipid) membrane.

The internal envelope was degraded by treatment with ether followed by trypsin but not by the reverse sequence or by either agent alone. From this and other experiments it was deduced that the internal envelope subunits are protein covered by lipid, the major portion of which is phosphoglyceride and triglyceride. The simplest model of the envelope consistent with the evidence comprises an internal envelope subunit protein associated with the polar ends of a layer of orientated phosphoglyceride molecules whose non-polar parts are in turn associated with a layer of triglyceride molecules. The residual space, bounded by adjacent subunits and by the external envelope, is occupied by cholesterol.


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