The dynamin family of proteins contains unique GTPases involved in membrane fission and fusion events throughout the cell. Dynamin is necessary for internalizing essential nutrients, is tightly coupled to cell signaling events, and has been linked to neuropathies and myopathies. As a vesicle invaginates, dynamin forms a spiral around the neck of the vesicle generating dynamin-lipid tubes that constrict and twist upon GTP hydrolysis, causing the vesicle neck to break (play video on top). The ability of dynamin to constrict mechanically the underlying lipid bilayer makes it unique among GTPases as a mechanochemical enzyme. However, the mechano-chemical processes governing the operation of dynamin at the molecular level are still under debate.
In collaboration with the laboratory of Prof. Vadim Frolov, we are using optical tweezers to measure the real-time activities of Dynamin 1 and Dynamin 2 proteins as they constrict single membrane nanotubes, in order to decipher the mechano-chemical events ruling out the operation of these enzymes.
‘In the meantime’, together with the labs of V. Frolov and A. Daga, we have uncovered the mechanism of ER membrane fission promoted by reticulon. The endoplasmic reticulum (ER) is a continuous cell-wide membrane network. Network formation has been widely associated with proteins producing membrane curvature and fusion, such as reticulons and atlastin. Regulated network fragmentation, occurring in different physiological contexts, is less understood. We found that the ER network has an embedded fragmentation mechanism based upon the ability of reticulons to produce fission of elongating network branches. In Drosophila, fission is counterbalanced by atlastin-driven fusion, with their imbalance leading to ER fragmentation. Live imaging of ER network dynamics upon ectopic expression of Drosophila reticulon linked fission to augmented membrane friction. Consistently, our single-molecule experiments with optical tweezers revealed that purified reticulon produced velocity-dependent constriction and fission of lipid nanotubes pulled from a flat reservoir membrane. Fission occurred at elongation rates and pulling force ranges intrinsic to the ER network, thus suggesting a novel principle of organelle morphology regulation where the dynamic balance between fusion and fission is governed by membrane motility.