Insertion of capsid proteins from nonenveloped viruses into the retroviralbudding pathway

Retroviral Gag proteins direct the assembly and release of virus particles from the plasma membrane. The budding machinery consists of three small dom ains, the M (membrane-binding), I (interaction), and L (late or "pinching-o ff") domains. In addition, Gag proteins contain sequences that control part icle size. For Rous sarcoma virus (RSV), the size determinant maps to the c apsid (CA)-spacer peptide (SP) sequence, but it functions only when I domai ns are present to enable particles of normal density to be produced. Small deletions throughout the CA-SP sequence result in the release of particles that are very large and heterogeneous, even when I domains are present. In this report, we show that particles of relatively uniform size and normal d ensity are released by budding when the size determinant and I domains in R SV Gag are replaced with capsid proteins from two unrelated, nonenveloped v iruses: simian virus 40 and satellite tobacco mosaic virus, These results i ndicate that capsid proteins of nonenveloped viruses can interact among the mselves within the context of Gag and be inserted into the retroviral buddi ng pathway merely by attaching the M and L domains to their amino termini, Thus, the differences in the assembly path,ways of enveloped and nonenvelop ed viruses may be far simpler than previously thought.