In plants, PIN proteins serve as secondary active transporters in the cell membrane that regulate the influx and efflux of auxins. They play an important role in the transport and production of auxins.[1]
Although PGP proteins also work as auxin transporters, it has been found that PIN proteins still function with or without PGP proteins. Moreover, yeast, which does not contain PGP, solely uses the PIN2 protein for auxin transport.[2]