The matrix protein of HIV-1 is not sufficient for assembly and release of virus-like particles.

The matrix (MA) proteins of human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV) are known to be important for the targeting and assembly of lentiviral proteins. The objective of the present study was to determine whether the MA protein of HIV-1 was sufficient for particle assembly and release. Eukaryotic expression of wild-type HIV-1 Gag-Pol, HIV-1 MA alone, or SIV MA alone was analyzed with radio-immunoprecipitation, density centrifugation, and a protease protection assay. Cells that expressed HIV-1 Gag-Pol or SIV MA alone released virus-like particles (VLPs) with sucrose gradient densities of 1.15 or 1.12 g/ml, respectively. The MA and/or capsid proteins in these particles were protected from protease degradation, indicating the presence of a protective outer membrane. Expression of HIV-1 MA protein alone resulted in release of MA which pelleted through a 20% sucrose cushion but failed to enter a 20-60% sucrose gradient and was not protected from protease degradation. The MA protein of SIV was previously reported to be sufficient for production of VLPs (S. A. Gonzalez, H, K, Affrachino, H. R. Gelderblom, and A. Burney. Virology 194, 548-556, 1993; V. Liska, D. Spehner, M. Mehtali, D. Schmitt, A. Kirn, and A. M. Aubertin. J. Gen. Virol. 75, 2955-2962, 1994). Our study confirmed that result, but indicated that the MA protein of HIV-1 was not sufficient to assemble and release VLPs.