Abstract. Nascent pectin and glucuronoarabinoxylan, synthesised in vitro by membrane-bound enzymes from etiolated pea Pisumsativum L.) epicotyls, were found to bind to pea xyloglucan in a pH-dependent manner. The binding was maximum at low pH 3±4), and decreased to almost zero at pH 6. The binding was probably non- covalent and reached saturation within 5 min. Removal ofthefucoseresiduesofxyloglucandecreasedthedegree of binding. Removal by protease of the proteins attached to nascent pectin and glucuronoarabinoxylan greatly reduced the maximum binding and abolished the pH-dependence. The observed binding may be of considerable signi®cance in the process of cell-wall assembly and in the control of cell extension. Key words: Binding pectin) ± Glucuronoara- binoxylan±Glycoprotein±Pectin± Pisum ± Xyloglucan Introduction The plant cell wall contains cellulose micro®brils embedded in a non-crystalline matrix. Matrix polysa- charides are synthesised in the Golgi apparatus and transported in vesicles to the wall Staehelin and Moore 1995). There, the newly synthesised matrix polysaccha- rides associate with each other and with newly synthes- isedcellulosetoformthenew,innermostlayerofthecell wall. The xylans are matrix polysacharides present abun- dantly in both monocots and dicots. Glucuronoarabin- oxylanGAX),presentindicotprimarywalls,containsa b-1-4-linked xylose backbone, to which a-1-2-linked 4-O-methyl glucuronic acid residues and 3-linked arabi- noseresiduesareattached.SynthesisofGAXinvolvesat least three enzymes, a xylosyltransferase, a glucuronyl- transferase, and an arabinosyltransferase. The ®rst two enzymes are located mainly in the Golgi apparatus Hobbs et al. 1991; Baydoun and Brett 1997). A 36- to 45-kDa protein is strongly, probably covalently, bound to nascent GAX in the Golgi Crosthwaite et al. 1994). The role of this protein is unknown. Pectin is abundant in the primary walls and middle lamellae of dicots. Its backbone contains homogalactu- ronan and the rhamnose- and galacturonic acid-con- taining main chain of rhamnogalacturonan 1 RG1). Shorter neutral side-chains containing galactose and arabinose are linked to rhamnose in RG1. Pectin synthesis involves one or more galacturonyltransferases, together with galactosyl-, arabinosyl-, xylosyl-, methyl-, acetyl- and rhamnosyltransferases Doong and Mohnen 1998; Baydoun et al. 1999). Xyloglucan, the principal hemicellulose of dicot primary walls, consists of a backbone of b-1-4-linked glucose residues, 60±75% of which have xylose residues attached to their carbon-6 Joseleau and Chambat 1984). Galactose residues are b-1-2)-linked to 30±35% of these xyloses. In primary wall xyloglucan, a fucose residue is a1-2)-linked to galactose Hayashi 1989). In seed storage xyloglucans, this fucose is absent. The mechanisms by which newly synthesised matrix polymers are incorporated into the wall are unclear. Xyloglucans are known to hydrogen-bond to cellulose Hayashi et al. 1994), and covalently bind to existing wall-xyloglucan through transglycosylation Thompson et al. 1997). Xylans also hydrogen-bond to cellulose Jarvis 1992). Recent studies have revealed a pH- dependent, non-covalent binding of nascent GAX to cell wall preparations from pea epicotyls and to hemicellulose extracted from the walls Brett et al. 1997). We now report the binding of nascent GAX and pectin to pea xyloglucan, and we describe the dependence of this binding on acid pH, the fucose residues of xyloglucan, and proteins attached to the nascent polysaccharides. Abbreviations: GalA = D-galacturonic acid; GAX = glucuro- noarabinoxylan; GlcA = D-glucuronic acid; GTC = guanidium thiocyanate; Xyl = xylose Correspondence to: C. T. Brett; E-mail: c.brett@bio.gla.ac.uk; Fax: +44-141-330-4447 Planta 2000) 211: 423±429 Protein- and pH-dependent binding of nascent pectin and glucuronoarabinoxylan to xyloglucan in pea Sandra E. Rizk 1,2 , Roula M. Abdel-Massih 1,2 , Elias A.-H. Baydoun 2 , Christopher T. Brett 1 1 Plant Molecular Science Group, Institute of Biomedical & Life Sciences, University of Glasgow, UK 2 Department of Biology, American University of Beirut, Beirut, Lebanon Received: 4 November 1999 /22 December 1999