Researchers in Berlin and Hannover have published a new structure of the protein dynamin in the journal Nature. The structure reveals how dynamin building blocks can form larger assemblies. It also shows how mutations that cause certain genetic diseases disrupt dynamin regulation.
Many molecules can't cross cell membranes without the help of vesicles – bubbles of membrane which transport large molecules from the outside to the inside of cells.
To form a vesicle, the cell membrane bends inwards until it is shaped like a bottle with a round body and a narrow neck. Dynamin wraps around the neck like a molecular lasso and uses energy to tighten and cut it. Cutting the neck releases the vesicle to be used inside the cell, for example it might be delivered to a specific location and opened to release its contents.
Mutations to dynamin, even those that affect only single amino acids in the protein, can cause disease by making dynamin more or less active. Centro-nuclear myopathy is one of the heritable diseases caused by mutations to dynamin and patients suffer progressive muscle weakness.
Oliver Daumke leads a group of researchers at MDC who focus on the structure of proteins that change the shape of cell membranes, including dynamin. They solved the structure of dynamin in collaboration with scientists at the Institute for Biophysical Chemistry of Hannover Medical School (MHH), the Freie Universität Berlin and the Leibniz-Institut für Molekulare Pharmakologie (FMP).
“We wanted to understand how dynamin is controlled so that is only active when and where it is needed,” explains Daumke. “For dynamin to work properly, it has to bind specifically to the necks of vesicles, and not to any other part of the cell membrane.”
Dynamin exists in cells in bundles of four dynamin molecules known as tetramers. At the necks of vesicles it assembles into a much larger helix that winds around the vesicle neck.
To understand how dynamin assembles, the researchers solved a crystal structure of the dynamin tetramer.
“Our structure of the dynamin tetramer has a bent shape,” says Katja Fälber, a lead author of the study. “It's a perfect match to the curvature of the necks of vesicles, which explains why dynamin assembles specifically there and not at flat membrane surfaces.”
The tetramers also act as building blocks that can fit together to form a dynamin helix. Dynamin molecules stick together in the tetramer via three contact sites. All these sites are within the stalk region of dynamin. The same contacts allow tetramers to assemble together into a helix. “For the first time we had the structural information to understand how the dynamin helix forms,” Fälber explains.
Dynamin: An expert at self-control
Dynamin is only active when it forms helical assemblies and this process is tightly regulated so that it only assembles at the necks of vesicles. “Dynamin is an expert at self control,” says Daumke. “It stops itself from forming large assemblies in a process called auto-inhibition.”
Dynamin practises auto-inhibition using its pleckstrin-homology (PH) domain. The PH domain binds to the stalk region and stops dynamin molecules assembling. The PH domain is also the part of dynamin that binds to membranes, so auto-inhibition is released when dynamin binds to vesicle necks. "The contact sites for helix formation only become accessible when dynamin binds membranes," says Daumke, “and we can see this for the first time in the structure of the tetramer”.
Interestingly, the interface between the PH domain and the stalk is where many of the mutations that cause centro-nuclear myopathy are found. This means that they could interrupt auto-inhibition and stop dynamin from being regulated properly.
“Knowing that disease-causing mutations are located at a site that's important to dynamin regulation gives us an insight into the molecular mechanism of disease,” Daumke says. In the future this insight can be applied to research aimed at treating centro-nuclear myopathy.
Reubold T.F., Faelber K, Plattner N, Posor Y, Branz K., Curth U, Schlegel J, Anand R, Manstein D.J., Noé F, Haucke V, Daumke O, & Eschenburg S.. Crystal structure of the dynamin tetramer. Nature 525(7569):404-8 (September, 2015). doi:10.1038/nature14880