Organization of immature human immunodeficiency virus type 1

Immature retrovirus particles contain radially arranged Gag polyproteins in which the N termini lie at the membrane and the C termini extend toward th e particle's center. We related image features to the polyprotein domain st ructure by combining mutagenesis with cryoelectron microscopy and image ana lysis. The matrix (MA) domain appears as a thin layer tightly associated wi th the inner face of the viral membrane, separated from the capsid (CA) lay er by a low-density region corresponding to its C terminus. Deletion of the entire p6 domain has no effect on the width or spacing of the density laye rs, suggesting that p6 is not ordered in immature human immunodeficiency vi rus type 1 (HIV-1). In vitro assembly of a recombinant Gag polyprotein cont aining only capsid (CA) and nucleocapsid (NC) domains results in the format ion of nonenveloped spherical particles which display two layers with densi ty matching that of the CA-NC portion of immature HIV-1 Gag particles. Auth entic, immature HIV-1 displays additional surface features and an increased density between the lipid bilayers which reflect the presence of gp41. The other internal features match those of virus-like particles.