Differential effects of brefeldin A and cycloheximide on the activity of auxin efflux carriers in Cucurbita pepo L.

Brefeldin A (BFA), an inhibitor of Golgi-mediated protein traffic, stimulat ed net uptake of indole-3-acetic acid (IAA) by etiolated zucchini (Cucurbit a pepo L.) hypocotyl tissue with a lag of less than 10 min, substantially r educed the rate of efflux of IAA from pre-loaded zucchini tissue, and stron gly inhibited polar transport of IAA in zucchini hypocotyl and pea internod e (Pisum sativum L.) segments. Pre-treatment of zucchini tissue with BFA (6 0 min), but nor its simultaneous addition to the uptake medium, also greatl y increased the: FCCP-sensitive uptake of IAA by microsomal vesicles prepar ed from the pre-treated tissue. In contrast to the effects of BFA, applicat ion of the translation inhibitor cycloheximide (CH) did nor affect any of t he parameters of IAA transport investigated fur at least 80 min and did not alter the responses to BFA. Nevertheless, CH rapidly (<10 min) reduced the incorporation of L-[S-35] methionine into TCA-insoluble cell proteins and into microsomal membrane proteins. Together these results provide strong ev idence that an essential protein component of the auxin efflux carrier syst em, targeted to the plasma membrane through the Golgi-mediated secretory sy stem, turns over rapidly ill the plasma membrane without a requirement for concurrent protein synthesis.