671, McConnell and Tyska show that microvillar myosin-1a pulls membrane for shedding along the core actin bundle.
Myosin-1a was first seen in micrographs 30 years ago bridging the actin bundle and the overlying membrane in microvilli. But it was not until 1989 that myosin-1a was characterized as an actin plus end–directed motor. Its potential as an active motor in the microvillus, rather than mere structural element, was overlooked until now.
A motor is useful in isolated brush borders, which dramatically shear off their membranes from the base to the tip of the microvillar stalks when fed ATP. McConnell and Tyska find that this actin plus end–directed shearing results in vesicles' being shed from the microvilli tips. The ATP activated shedding by turning on myosin-1a: brush borders from myosin-1a knock-out mice shed membrane at only five percent of the wild-type level.
How myosin-1a gains traction on the membrane surface is an open question. But the discovery promotes the brush border from merely a static scaffold of boosted surface area for nutrient breakdown to an active zone for shedding vesicles—tiny packets that may speed nutrient breakdown, shed invading microbes, send messages, and improve membrane remodeling.