The A17L gene product of vaccinia virus is exposed on the surface of IMV

The p21 membrane protein of vaccinia virus (VV), encoded by the A17L gene, has been reported to localize on the inner of the two membranes of the intr acellular mature virus (I MV). It has also been shown that p21 acts as a me mbrane anchor for the externally located fusion protein p14 (A27L gene). Si nce p14 is located on the surface of IMVs, it is hard to envision that p21 should be located only on the inner membrane. Our results from (i) immunoel ectron microscopy, (ii) biotinylation, and (iii) protease treatment of puri fied IMVs showed that the N-terminus of p21 is exposed on the surface of vi rus particles, while the C-terminus is embedded in the membrane. Mono-speci fic antibodies to the N-terminus of p21 neutralize infection of VV while an tibodies to the C-terminal domain do not. We suggest that p21 molecules are located both in the inner and in the outer membrane of IMV. (C) 2001 Acade mic Press.