Role of Arabidopsis p-glycoproteins and the membrane associated aminopeptidase APM1 in regulating cellular auxin transport

The purpose of the present study was to investigate the functions of the Arabidopsis p-glycoproteins and the membrane associated aminopeptidase APM1 in regulation of cellular auxin transport. Polar transport of the plant hormone auxin is essential for the establishment and maintenance of plant polarity. Regulation of polar auxin transport involves intricate interactions between multiple components of membrane transport and cellular trafficking complexes. The auxin efflux inhibitor 1-naphthylphthalamic acid (NPA) acts on regulatory sites in these complexes and affects auxin transport and vesicular trafficking. In Arabidopsis, p-glycoprotein (PGP) ABC transporters have been shown to exhibit higher affinity NPA binding while an M1 membrane aminopeptidase APM1 was shown to exhibit low affinity binding. PGPs and members of the PIN (from pinformed inflorescence) subfamily of ion-coupled major facilitator proteins have been shown to transport auxin independently. Flavonoids, the endogenous regulators of auxin transport, inhibit transport primarily at the root and shoot tip. PIN localization is altered in flavonoid deficient mutants and appears to be modulated by developmental cues. PGPs are likely to be the principal targets of flavonoid regulation at the plasma membrane. Although PINS and PGPs can transport auxin independently they interact in planta to confer specificity, inhibitor sensitivity and transport directionality. PIN mislocalization in pgp19 mutants suggests that PGPs are involved in stabilizing PINs on the plasma membrane. PGP19 and PIN1 membrane localization overlaps in specific tissues and co-immunoprecipitation and yeast two hybrid studies indicate direct interaction between the two. APM1 is a component of the vesicular trafficking pathway and it appears to mediate regulation of PIN protein trafficking. It plays an essential role in Arabidopsis root development. The root defects observed in apm1 mutants can be traced back to the late globular stage of embryogenesis. Aminopeptidase inhibitors that disrupt cellular trafficking phenocopy apm1 mutants. High concentrations of NPA also phenocopy these defects. APM1 is auxin responsive and responds to changes in endogenous auxin levels in a tissue specific manner. Like mammalian dual function M1 aminopeptidases, APM1 appears to function in both processing and trafficking of membrane proteins