"Auxin molecules essentially are pulled through the cell membrane by PGP transport proteins," Murphy said. "It's an energetic process that happens like pulling a rope through something sticky."
Both the multi-drug resistant PGPs in people and plants are part of a large family of proteins, called ATP-binding cassette (ABC) proteins, that act as delivery trucks to detoxify cells, send messages from cell to cell to influence biochemical reactions, and to regulate those reactions. The ABC proteins are so named because they must bind with ATP, the main cell energy source, in order to fulfill their mission.
The best known member of another class of transport proteins, PIN1, also may be a transporter, but appears to function primarily as an aide rather than the delivery truck for auxin transport, Murphy said. This finding revealed that PINs and PGPs may function together in long-distance auxin transport, according to the Plant Journal article. Named for the pin-shaped appearance of the mutant originally used to identify the gene that directs the activities of PIN1, these proteins are members of the major protein family, called facilators, that aid processes such as hormone transport.
Recent evidence suggests that teamwork between PGP and PIN proteins determines the direction auxin moves and, therefore, how the plant develops, Murphy said. In plants, shape, height and bending in response to light and gravity are largely determined by the direction and amount of auxin moving through their tissues.
Murphy and his collaborators on the Plant Journal study found that PGP1 and PGP19 move the hormone out of cells.
In the November Plant Cell report, Murphy's research team reported that another P-glycoprotein, PGP4, functions in the opposite direction, providing the boost needed to import the hormone auxin into cells and to increase the amount transported.
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