A basic structure that allows cells and subcellular units to converse in chemical language turns out to have a lot more abilities than expected. The structure, called a “fusion pore,” allows hormones, chemicals that carry nerve impulses, and proteins produced in containers inside cells to reach their destination. Without fusion pores, multicellular life is difficult to envision, says University of Wisconsin–Madison neuroscientist Edwin Chapman.
But Chapman says the previous view — that the pore works as a pipe — needs major revision. Based on research he published online today (Jan. 31) in the journal Nature, he argues that fusion pores are actually highly dynamic valves. A pore can be mostly open, and flicker closed. It can be mostly closed, but flicker open. Or it can have some in-between state.
Fusion pores are essential in the nervous system, since neurotransmitters like dopamine and serotonin rely on them to reach their target. “These pores connect membranes in all organisms more complicated than a bacteria, but they are a whole lot more complicated than a pipe,” Chapman says.
The new research shows that opening in a pore depends on how many SNARE proteins are in its structure, Chapman says.
SNAREs build fusion pores in all organisms containing a cell nucleus, a range that starts with many single-celled life forms and moves up through plants and animals to humans. That means both the pores and the SNARE proteins that construct them probably evolved more than 1 billion years ago.
And that means that the structure and function of fusion pores matters fundamentally in biology, says Chapman, a Howard Hughes Medical Institute investigator and professor of neuroscience at UW–Madison.
“Some folks think of a cell as a bag of aqueous goo, but in reality, it contains hundreds or thousands of organelles, each surrounded by its own membrane,” he says.
“All of these organelles contain or process or accept various substances, and respond to myriad signals. To emit or take up a substance, these containers need to build a fusion pore across membranes. These pores are all over the place whenever an organism has more than one cell.”
In 2013, the Nobel Prize was awarded for the discovery of SNAREs and the topic of membrane fusion. According to conventional wisdom, when a fusion pore forms, the SNARE proteins lock into place with a zipper-like action, creating a structure that essentially never closes — that is to say, they form a pipe, a “dumb” connection across normally impermeable membranes.
In his office, Chapman Googles “SNARE proteins,” and returns a screenful of images showing corkscrew-shaped molecules, intertwined as they seize the outer membranes of two cells. “They did not give us credit at Wikipedia, but we drew that cartoon,” he says, with delicious irony.
“And now we’ve proven that this model is wrong,” he says. “The textbooks need to be adjusted.”
The new research shows that the pores are sophisticated, sensitive and trigger-happy valves that can open and close thousands of times a second. “I expect we’re going to see a lot of rethinking,” Chapman says. “It’s one thing to have a pipe that’s jammed open. It’s quite another to have one that is mostly closed, but flickers open momentarily — or vice versa.”
SNAREs do not fully assemble into stable complexes before fusion, Chapman says. “Rather, SNAREs are zippering and unzippering and driving dynamic changes in fusion pores after they have opened.”
The new research relied on an apparatus devised by Chapman and his colleagues that can, for the first time, record exactly when the fusion pore opens and closes. Depending on the number of SNARE proteins in the pore, and its diameter, they have found that a pore may be almost entirely closed. It may be closed, but open for a burst every half second or so. It may do the opposite.
“This is unexpected flexibility, unexpected dynamics,” Chapman says. “Fusion pores have been thought of as something that is either open or closed. The signal passes, or it does not. But when we studied them at the microsecond time scale, we saw something quite unexpected".
"With three SNARE proteins present, they flicker open, but are mostly closed. With five SNAREs, most of the time they are open, but they transiently close. With seven SNARES, they are always open.”
Without membrane fusion enabled by these pores, many substances made in a cell or a subcellular unit would be trapped and useless, Chapman says. “Membrane fusion is a fundamental question for organisms with a nucleus,” a category that includes yeast.
“When one considers that the human brain contains on the order of 10 to the 15 synapses, an uncountable number of fusion pores form in our brains over the course of a day. Only now are we starting to understand the structure and dynamics of these mysterious nanoscale molecular machines.”
In the nervous system, fusion pores underlie signaling by transporting a chemical message across a narrow gap from one neuron’s axon to another neuron. Fusion pores are also fundamental in other aspects of biology, ranging from the release of insulin after a meal to the entry of viruses into target cells.
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Image credit: University of Wisconsin-Madison.