A model for [T] to [R] equilibrium in different conditions. Some elements are susceptible to influence the MA [T] vs [R] equilibrium, in the context of MA alone (in the mature particle, during the early step of infection, or in vitro), or as a domain of the Gag polyprotein. The "initial" equilibrium (in solution, purified protein, concentration around 1 μM) between the [T] and [R] conformations of HIV (A) and MLV (B) MAs (a) or Gag (b) are depicted, the size of the protein representing the relative amount of each form. The factors susceptible to induce a majority of a given conformation are written in bold characters. Others, such as PI(4,5)P2 in the case of HIV-MA, are only able to (slightly) displace the equilibrium, even at a saturating concentration (Aa).