Re-evaluation of the role of auxin binding site II

Binding of 1-naphthylacetic acid (1-NAA) was assayed in microsomal membranes from Zea mays coleoptiles and from hypocotyls of Cucurbita pepo. Auxin binding site II was differentiated from site I binding by using phenylacetic acid (PAA) to saturate site I binding capacity. The amount of type-II binding sites, per gram original fresh weight, was 34 pmol with Zea and 6.4 pmol with Cucurbita. When maize membranes were separated by dextran gradient centrifugation, auxin binding site II migrated coincident with tonoplast marker enzymes. The physiologically active auxin 4-chloroindoleacetic acid (4-Cl-IAA) competed very poorly with 1-NAA binding to both site I and site II. This result suggests that sites I and II are not involved in the regulation of growth. When comparing isolated outer epidermis with intact coleoptile of Zea, similar amounts and ratios of site I and site II binding activities were observed.