Figure 1.

A schematic representation of retrovirus budding. (a) Gag proteins move to the plasma membrane and begin to associate with one another. (b) Formation of electron-dense aggregates under a deforming plasma membrane follows. (c) Bud curvature steadily increases. (d) Membrane fusion leads to pinching-off of the virion; (e) proteolytic processing of Gag leads to virion maturation and formation of an electron dense core. L-domain mutants of most retroviruses arrest at a stage equivalent to (d) but with an extended stalk [3,4]; in other viruses, such as human T-lymphotropic virus 1 (HTLV-1), arrest occurs at a stage roughly equivalent to (b) [38]. MA, matrix; CA, capsid; NC, nucleocapsid; Env, envelope proteins.