Dynamin was, early on, localized to the collar around the neck of forming endocytic vesicles. This suggested that dynamin may use the energy of GTP hydrolysis to directly pinch a membranous neck. Indeed, dynamin could tubulate lipids and break apart the tubules in vitro, although later it seemed that the breaking apart was happening as the samples dried on EM grids.
Meanwhile, Sandy Schmid (Scripps Research Institute, La Jolla, CA) had come up with a “regulatory GTPase” model: that dynamin was active not as it hydrolyzed GTP but in its GTP-bound form, which recruited other proteins to do the pinching. This theory was controversial, and now the Yale group has more evidence for the earlier “pinchase” model.
They used light microscopy rather than EM to follow tubulation and fission directed by dynamin in vitro. Addition of GTP caused twisting, retraction, and, if both ends were anchored, straightening and breakage. Thus longitudinal tension was needed with constriction to achieve fission.
Twisting was evident because tubules became supercoiled and, most strikingly, attached beads rotated around the tubules. “You can't look at that movie and say there isn't something mechanochemical going on,” says Schmid. Previous work had established the importance of a conformational change in dynamin, but “that doesn't distinguish between mechanochemical and regulatory functions,” says Schmid. “Spinning around a liposome sure does.”
Not that Schmid is throwing out the regulatory model. “The two models are not mutually exclusive,” she says. “This is a smarter molecule than we give it credit for. There is a lot more to this molecule than a pinchase.” She thinks the regulatory model may be operative early on, as dynamin assesses curvature, coated pit formation, or cargo loading before giving the go-ahead.
The “pinchase” function of dynamin would then take over. In vivo, say Roux and de Camilli, the dynamin collars are relatively short, so cortical actin is the most likely source of tension that would help the dynamin to wrench an endocytic vesicle free.