The human immunodeficiency virus type-1 matrix (HIV-1 MA) is best described as a multi-functional, structural protein. However, the multitude of functional activities ascribed to this viral component is not nearly as interesting as are its seemingly paradoxical and opposing roles during the viral life cycle. At the time of virus infection, HIV-1 MA remains associated with the reverse transcription complex, in which viral nucleic acids are synthesized, and facilitates its translocation to the host cell nucleus (Bukrinsky, Sharova et al. 1992; Bukrinsky, Sharova et al. 1993). This activity of MA has been proposed to form the basis for the infection of non-dividing cells (Bukrinsky, Haggerty et al. 1993). An interaction between the C-terminally phosphorylated form of MA and HIV-1 integrase, an integral component of the complex, was initially proposed to mediate this association (Gallay, Swingler et al. 1995; Gallay, Swingler et al. 1995). However, conditions which promote dissociation of integrase from the reverse transcription complex do not reduce MA association (Miller, Farnet et al. 1997). The possibility of a direct interaction between MA and the viral genome is discussed in Chapter III. The nucleophilic nature of HIV-1 MA is paradoxical with its reported activity in targeting the viral precursor proteins to the cytoplasmic membrane (Krausslich and Welker 1996), during the particle production phase of the viral life cycle. Furthermore, MA when expressed in the absence of other viral proteins exhibits a cytoplasmic localization (Fouchier, Meyer et al. 1997); a result which does not support a nuclear translocation role for this protein. The work presented here resolves this seemingly controversial issue. We demonstrate that MA exhibits a strong nuclear export activity. This newly discovered activity is designed to effectively counteract the protein's innate nucleophilic nature, thus maintaining a cytoplasmic localization. The nuclear export function of MA is sensitive to changes within the conformation of the protein as C- and N-terminal deletions, as well as point mutations in the protein, abolish the activity. Furthermore, the export activity is mediated by the Crm1 NES receptor (Fornerod, Ohno et al. 1997; Fukuda, Asano et al. 1997; Ossareh-Nazari, Bachelerie et al. 1997) despite the lack of a leucine-rich export signal within the matrix coding region. Therefore, the interaction between matrix protein and Crm1 is most likely to be mediated by another, perhaps cellular, protein. Any changes in matrix structure may lead to the disruption of this protein-protein interaction. We discuss a model implicating a phosphorylation event in the inactivation of this nuclear export signal. An even more fascinating issue regards the role of this nuclear export activity, during the viral life cycle, and is detailed in Chapter II. In short, mutations in MA which impair its nuclear export activity result in nuclear accumulation of the precursor Gag polyprotein (Pr55) and the nucleocapsid-associated viral genomic RNA. As a result, non-infectious virions deficient in genomic viral RNA are produced. Therefore, drugs designed to block this export activity can undermine the carefully orchestrated course of events during HIV replication and can shut down the growth of the virus.