Shape transformations of active tubular membranes

Motivated by the action of enzymes that flip lipid molecules from one monolayer to another in a lipid bilayer membrane, we study shape instabilities of a tubular membrane driven by lipid-flipping. We begin with the instability of a tube with a fixed lipid number density distance, determining the relative importance of solvent viscosity, membrane viscosity, and bilayer friction. Then we consider the case of a uniform density of enzymes acting at a fixed rate. Implications for experiments will be discussed.