MEMBRANE TUBULATION AND PROTON PUMPS

A model is presented for one type membrane tubulation. In this model large transmembrane protein complexes link together to form helical bands. To form the helical bands the individual complexes within the band would need to form a slight bend and twist with adjacent complexes as they link together. Such linkages would lead to the band and lipid bilayer extending out from the plane of the membrane to form a tube of a diameter equal to or smaller than the radial diameter of the helix. This model is supported by two examples of membrane tubulation found in Paramecium, in which proton pumps are the transmembrane complexes that are associated together to form the helically coiled bands. These include the helically linked F0F1 complexes of the mitochondrial inner membrane and the V0V1 complexes of the decorated tubules of the contractile vacuole complex. Such tubules result in very high surface-to-volume ratios and may be important in efficient ATP production or in forming proton gradients for transporting ions across membranes.