Bacteria have plenty of things to send out into world beyond their own boundaries: coordinating¹ signals to other members of their species, poisons for their enemies, and devious² instructions to manipulate host cells they have infected. Before any of this can occur, however, they must first get the shipments past their own cell membranes⁴, and many bacteria have evolved specialized⁵ structures and systems for launching the proteins that do these jobs. Researchers at The Rockefeller University have determined⁶ the structure of a simple but previously⁷ unexamined pump that controls the passage of proteins through a bacterial⁸ cell membrane³, an achievement that offers new insight into the mechanics that allow bacteria to manipulate their environments. The results were published in Nature on July 23.

 

"This pump, called PCAT for peptidase-containing ATP-binding cassette transporter, is composed of a single protein, a sort of all-in-one machine capable of recognizing its cargo⁹, processing it, then burning chemical fuel to pump that cargo out of the cell," says study author Jue Chen, William E. Ford¹⁰ Professor and head of the Laboratory of Membrane Biology and Biophysics. "This new atomic-level structure explains for the first time the links between these three functions."

 

Of the many types of molecules¹² cells need to move into and out of their membranes, proteins are the largest. PCATs specialize in pumping proteins out of the cell, and, because they are single-molecule¹¹ machines that work alone, or with two partner proteins in some bacteria, they are the simplest such systems.

 

Each PCAT molecule has three domains¹³, each in duplicate: one recognizes the cargo by a tag it carries, and cuts off that tag; another binds¹⁴ to and burns ATP, a molecule that contains energy stored within its atomic bonds; and the third forms a channel that spans the cells membrane. Previous work had examined the structure of the first two domains, but the structure of the third, had remained a mystery, along with the details of how the components¹⁵ function together.

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