Differential distribution of xyloglucan glycosyl transferases in pea Golgi dictyosomes and secretory vesicles

Abstract

ABSTRACT Rate-zonal centrifugation of pea (Pisum sativum var. Alaska) stem microsomal membranes on a linear Renografin gradient separated Golgi secretory vesicles from dictyosomes. Secretory vesicles possessed high levels of xyloglucan fucosyl transferase activity, which effects the final decoration of stem xyloglucan side-chains, but lacked substantial xyloglucan xylo-syl transferase activity, which is required for the synthesis of the xyloglucan backbone. In contrast, total dictyosomal membranes possessed both fucosyl and xylosyl transferase activities. Isopycnic centrifugation of a homogenized dictyosome-enriched membrane preparation on a shallower Renografin gradient indicated that lighter dictyosomal membranes possessed xylosyl transferase but relatively little fucosyl transferase activity. The bulk of the dictyosomal membranes formed a denser peak in which xylosyl and fucosyl transferase activities codistributed. Thus a differential localization of function in the Golgi apparatus during biosynthesis of xyloglucan is indicated. A tentative mechanism is suggested in which the elaboration of the glucose-xylose backbone is initiated in lighter dictyosomal membranes, backbone synthesis is concluded and fucosylation begun in denser dictyosomal membranes, and fucosylation completed in Golgi secretory vesicles during transport of xyloglucan to the cell wall.