The human immunodeficiency virus (HIV) matrix protein, p17, forms the outer shell of the core of the virus, lining the inner surface of the viral membrane. The protein has several key functions. It orchestrates viral assembly via targeting signals that direct the gag precursor polyprotein, p55, to the host cell membrane and it interacts with the transmembrane protein, gp41, to retain the env-encoded proteins in the virus. In addition, p17 contains a nuclear localization signal that directs the preintegration complex to the nucleus of infected cells. This permits the virus to infect productively non-dividing cells, a distinguishing feature of HIV and other lentiviruses. We have determined the solution structure of p17 by nuclear magnetic resonance (NMR) with a root-mean square deviation for the backbone of the well-defined regions of 0.9 A. It consists of four helices connected by short loops and an irregular, mixed beta-sheet which provides a positively charged surface for interaction with the inner layer of the membrane. The helical topology is unusual; the Brookhaven protein database contains only one similar structure, that of the immune modulator interferon-gamma.

The virus‐like particles (VLPs) of the yeast retrotransposon Ty are genetically, structurally and functionally analogous to retroviral nucleocapsids or cores. Like retroviral cores Ty‐VLPs package and possibly promote the enzyme activities for reverse transcription and integration, as well as encapsulating the RNA that is the intermediate in retrotransposition. Here we show that Ty‐VLPs assemble into symmetrical structures across a broad distribution of particle sizes. This spread of sizes violates the principle of quasi‐equivalent packing. In addition, RNase accessibility experiments suggest that these particles form an open structure that does not protect the encapsulated RNA. These features distinguish Ty‐VLPs from typical spherical viral capsids in both structure and function.

We present an immunological characterization of the Ty1 virus-like particle (VLP). A panel of monoclonal and polyclonal antibodies were raised against the TYA particle-forming protein. Using these antibodies in epitope availability assays two N-terminal regions of the TYA protein were mapped projecting from or at the surface of the proteinaceous shell of the VLP. Two different C-termini of the TYA protein, corresponding to the C-terminus of the full-length and truncated forms, were seen to be buried within the particle core and not available for antibody binding. RNase accessibility studies demonstrated a difference in the porosity of the protein shell surrounding the Ty1 nucleic acid between different particle types, suggesting differences in subunit organization.

The yeast retrotransposon, Ty1, produces a macromolecular structure known as a virus-like particle (VLP) as an essential part of its replication cycle. The Ty1 Gag-like structural protein TYA, p1-440, alone is capable of directing assembly of the VLP. In order to determine the TYA sequences required for assembly, we have produced a series of truncated and deleted TYA forms and assessed their ability to assemble into particles. Removal of 100 amino acids from the C-terminus renders the TYA protein, p1-340, incapable of particle assembly; however, p1-363 with 77 residues missing from the C-terminus is capable of assembly. Removal of 40 amino acids from the N-terminus (p41-440 and p41-381) does not affect particle formation but more severely N-truncated forms, p71-381 and p100-381, are present as large aggregates within the cells and are therefore either incapable of or unavailable for VLP formation. Analysis of an internally deleted TYA, p1-381 delta 62-114, has identified this as a possible region of the TYA protein important for subunit:subunit interactions during the particle assembly.