Interactions Between the Actin Cytoskeleton and an Auxin Transport Protein

Abstract

In shoots, polar auxin transport is basipetal (i.e., from the shoot apex toward the base), and is driven by the basal localization of the auxin efflux carrier complex. One mechanism by which this efflux carrier complex could be localized to the basal membrane is through attachment to the actin cytoskeleton. The efflux carrier protein complex is believed to consist of several polypeptides, including a regulatory subunit that binds auxin transport inhibitors such as naphthylphthalamic acid (NPA). Several lines of experimentation have been used to determine whether the NPA-binding protein interacts with actin filaments. The NPA-binding protein has been shown to partition with the actin cytoskeleton during detergent extraction. Agents that specifically alter the polymerization state of the actin cytoskeleton also change the amount of NPA-binding protein and actin recovered in these cytoskeletal pellets. Actin affinity columns were prepared with polymers of actin purified from zucchini hypocotyl tissue. NPA-binding activity was eluted in a single peak from the actin filament column. Cytochalasin D, which fragments the actin cytoskeleton, was shown to reduce polar auxin transport in zucchini hypocotyls. The interaction of the NPA-binding protein with the actin cytoskeleton may localize it in one plane of the plasma membrane, and thereby control the polarity of auxin transport.