PIN proteins
In plants, the PIN proteins are integral membrane proteins that transport the anionic form of the phytohormone auxin across membranes. Most of the PIN proteins localize at the plasma membrane where they serve as secondary active transporters involved in the efflux of auxin. The PM-localized PIN proteins show asymmetrical localisations on the membrane and are therefore responsible for polar auxin transport. Some other members of the PIN family localize mostly at the ER-membrane or have a dual PM and ER localisation. These PIN proteins regulate the partitioning of auxin within the cell.
The PM-localized PIN proteins physically interact with a few members of the large PGP family of transporters that also work as auxin efflux carriers. These interactions result in a synergistic increase in auxin efflux.
The activity and localization of the PM-localized PIN proteins is regulated by several phosphorylations on their large cytosolic hydrophilic loop carried out by kinases of the AGC family and the D6PK kinase.
