The stability of a flexible fluid membrane containing a distribution of
mobile, active proteins (e.g. proton pumps) is shown to depend on the structure
and functional asymmetry of the proteins. A stable active membrane is in a
nonequilibrium steady state with height fluctuations whose statistical
properties are governed by the protein activity. Disturbances are predicted to
travel as waves at sufficiently long wavelength, with speed set by the normal
velocity of the pumps. The unstable case involves a spontaneous, pump-driven
undulation of the membrane, with clumping of the proteins in regions of high
activity.Comment: 4 two-column pages, two .eps figures included, revtex, uses eps