A PUTATIVE ALPHA-HELICAL STRUCTURE WHICH OVERLAPS THE CAPSID-P2 BOUNDARY IN THE HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 GAG PRECURSOR IS CRUCIAL FOR VIRAL PARTICLE ASSEMBLY

The capsid (CA) and nucleocapsid domains of the human immunodeficiency virus type I Gag polyprotein are separated by the p2 spacer peptide, which is essential for virus replication, Previous studies have reveal ed that p2 has an important role in virus morphogenesis, In this paper , we show that a crucial assembly determinant maps to the highly conse rved N terminus of p2, which is predicted to form part of an alpha-hel ix that begins in CA, A mutational analysis indicates that the ability of the N terminus of pi. to adopt an alpha-helical structure is essen tial for its function during virus assembly, To prevent CA-p2 processi ng, it was necessary to mutate both the CA-p? cleavage site and an int ernal cleavage site within p2, Virions produced by the doable mutant l acked a conical core shell and instead contained a thin electron-dense shell about 10 nm underneath the virion membrane, These results sugge st that p2 is transiently required for proper assembly, but needs to b e removed from the C terminus of CA to weaken CA-CA interactions and a llow the rearrangement of the virion core shell during virus maturatio n.