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Cover  Remodeling of biological membranes is an important process utilized during membrane trafficking, cell movement, and organelle biogenesis. A host of different lipid-modifying enzymes, structural coat and cytoskeletal proteins, and molecular motor enzymes have been implicated in this process. One such mechanoenzyme is the large GTPase dynamin that is known to assemble into polymeric helical rings (Hinshaw and Schmid, Nature 374, 1995) and has been resolved as electron-dense collars that encircle invaginated membrane (Takei et al., Nature 374, 1995). Seminal studies by Sweitzer and Hinshaw (Cell 92, 1998), and later by Takei et al. (Cell 94, 1998), have demonstrated that dynamin can tubulate and subsequently vesiculate spherical liposomes through GTP hydrolysis. An example of this property is shown in the insert on the cover (provided by Jenny Hinshaw), where numerous polymeric ring-like striations can be seen constricting lipid tubules in the liposome assay. The dynamins constitute an extended family of enzymes that display significant homology within a conserved GTPase domain, suggesting that other family members may have similar membrane remodeling activity. A dynamin-like protein, DLP1/DRP1, which shares greater homology to the yeast dynamin family member, Dnm1p (42%), than to conventional dynamins (38%), has recently been identified in mammalian cells (Yoon et al., J. Cell Biol. 140, 1998) and is predicted to function as a mitochondrial fission enzyme (Smirnova et al., Mol. Biol. Cell 12, 2001). In this month's cover illustration, Yoon et al. (Mol. Biol. Cell 12, 2001) have demonstrated that DLP1 also has the capacity to form polymeric rings and actively tubulates membrane. Thus, the dynamin family of proteins seems to share a function dedicated to the tubulation and deformation of biological membranes.Mark McNiven