PIN proteins

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PIN proteins are plant membrane proteins that move auxin, a key growth hormone, out of cells. The first PIN to be studied in detail, PIN1, was discovered because a pin1 mutation in Arabidopsis thaliana caused stems with almost no organs, including leaves and flowers.

Most PIN proteins (such as PIN1, PIN2, PIN3, PIN4, and PIN7) sit in the plasma membrane and act as secondary active transporters that export auxin from cells. Their placement on the membrane is asymmetric, which creates directional or polar auxin transport. Other PINs have different localizations: PIN5 and PIN8 mainly reside in the endoplasmic reticulum (ER), and PIN6 can be found in both the plasma membrane and ER. These PINs help control how auxin is distributed inside the cell.

PINs at the plasma membrane often work together with PGP transporters to boost auxin efflux. The activity and location of PIN proteins are controlled by phosphorylation from AGC family kinases (like PID, WAG1, WAG2, PID2) and the D6PK kinase.

In the membrane, PINs form clusters of varying sizes. These clusters influence signal processing, sensitivity, and trafficking inside the cell. Clustering helps maintain the polarity of transporters and affects how easily they move. Cluster formation and movement depend on phosphoinositides and enzymes such as PIP5K1. PIN clustering is influenced by, but not strictly tied to, REMORIN proteins, and can be increased by higher levels of salicylic acid. The composition of the cell wall (like pectin and cellulose) also shapes PIN clustering and thus auxin transport. The microtubule cytoskeleton further modulates how PINs diffuse along the membrane.

Auxin itself affects PIN polarity, though new research shows that both natural and synthetic auxins can promote endocytosis of PIN2 at low concentrations, a process that can help preserve polarity in some situations. Auxin signaling triggers transcriptional changes (involving factors such as WKY23) that regulate PINs, while a receptor system called CAMEL–CAR phosphorylates PINs to control their trafficking and polarity.

Besides transcriptional effects, auxin also triggers rapid, non-transcriptional responses that influence proteins like Myosin XI and its partner MadB2, contributing to fast developmental changes. Together, these processes coordinate polar auxin transport, guiding plant growth and development.