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Pectins of varying structures were tested for their ability to interact with cellulose in comparison to the well-known adsorption of xyloglucan. Our results reveal that sugar beet (Beta vulgaris) and potato (Solanum tuberosum) pectins, which are rich in neutral sugar side chains, can bind in vitro to cellulose. The extent of binding varies with respect to the nature and structure of the side chains. Additionally, branched arabinans (Br-Arabinans) or debranched arabinans (Deb-Arabinans; isolated from sugar beet) and galactans (isolated from potato) were shown bind to cellulose microfibrils. The adsorption of Br-Arabinan and galactan was lower than that of Deb-Arabinan. The maximum adsorption affinity of Deb-Arabinan to cellulose was comparable to that of xyloglucan. The study of sugar beet and potato alkali-treated cell walls supports the hypothesis of pectin-cellulose interaction. Natural composites enriched in arabinans or galactans and cellulose were recovered. The binding of pectins to cellulose microfibrils may be of considerable significance in the modeling of primary cell walls of plants as well as in the process of cell wall assembly.The well-known model of primary cell walls (PCWs) of dicotyledons emphasizes noncovalent interactions between cell wall polymers and suggests two independent, but interacting, networks where the cellulose-xyloglucan network is embedded in a matrix of pectic polysaccharides (Carpita and Gibeaut, 1993;Somerville et al., 2004).Cellulose, the primary structural element of the cell wall, is a homopolymer composed of (1/4)-linked b-D-Glcp residues. The linear chains of parallel alignment are tightly linked by hydrogen bonds to form microfibrils. Xyloglucan, the most abundant hemicellulosic polysaccharide in the PCWs of dicotyledons, is composed of a cellulose-like backbone consisting of (1/4)-linked b-D-Glcp residues, branched at O-6 by a-D-Xylp residues, which can be further substituted at O-2 by b-D-Galp residues (Fry, 1989). Some of the Galp residues may be substituted at O-6 by a-D-Fucp. Pectins are major components of dicotyledon PCWs and of the middle lamella. Their backbone is composed of smooth homogalacturonan (HG) and hairy rhamnogalacturonan (RG) I regions (O'Neill et al., 1990). HG, a linear chain composed of (1/4)-linked a-D-GalUAp units, can be methyl esterified at O-6 of carboxyl groups and acetyl esterified at O-2 and/or O-3 of secondary hydroxyl groups (Ralet et al., 2001). Some HGs might be substituted to form RG II or xylogalacturonan. RG II is a complex polysaccharide composed of GalUAp, Rhap, Galp, and some unusual sugars.Dimers of RG II were found to be cross-linked by two diester bonds through a boron atom (Fleischer et al., 1999). Xylogalacturonan contains b-D-Xylp residues attached to O-3 of the HG backbone (Le Goff et al., 2001). RG I contains a backbone of the repeating di- Renard et al., 1995). They are predominantly substituted at O-4 of Rhap residues by neutral sugar side chains (Schols and Voragen, 1994). The proportion of branched Rhap residues depen...

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The storage modulus G' of rennet and acid milk gels filled with milk fat globules was measured as a function of the fat globule surface composition (native milk fat globule membrane, caseins and whey proteins, or a mixture of the three due to mechanical treatments) and surface area (i.e., the fat globule size). By different technological procedures, it was possible to obtain fat globules of constant surface composition but various sizes, and vice-versa, which had never been done. For both rennet and acid gels, a critical fraction of the fat globule surface covered by caseins and whey proteins was identified (approximately 40%), beyond which G' increased. Below this threshold, the gel viscoelasticity was unaffected by mechanical treatments. When the diameter of native milk fat globules decreased, the G' of rennet gels increased slightly, whereas that of acid gels decreased sharply. For both types of gels, G' increased when the diameter of partially disrupted fat globules decreased. For recombined globules completely covered with caseins and few whey proteins, G' increased with fat globule surface area for rennet gels whereas it decreased for acid gels. With the help of confocal microscopy and in the light of general structural differences between rennet and acid gels, a mechanism is proposed for the effect of fat globule damage and diameter on G', depending on the interactions the globules can undergo with the casein network.

Uracil permease is a multispanning protein of the Saccharomyces cerevisiae plasma membrane which is encoded by the FUR4 gene and produced in limited amounts. It has a long N-terminal hydrophilic segment, which is followed by 10 to 12 putative transmembrane segments, and a hydrophilic C terminus. The protein carries seven potential N-linked glycosylation sites, three of which are in its N-terminal segment. Overexpression of this permease and specific antibodies were used to show that uracil permease undergoes neither N-linked glycosylation nor proteolytic processing. Uracil permease N-terminal segments of increasing lengths were fused to a reporter glycoprotein, acid phosphatase. The in vitro and in vivo fates of the resulting hybrid proteins were analyzed to identify the first signal anchor sequence of the permease and demonstrate the cytosolic orientation of its N-terminal hydrophilic sequence. In vivo insertion of the hybrid protein bearing the first signal anchor sequence of uracil permease into the endoplasmic reticulum membrane was severely blocked in sec6l and sec62 translocation mutants.Many specific yeast permeases have been subjects of genetic and physiological studies (8), while more recent reports have concentrated on their molecular biology (1,5,6,13,21,30,43,51). Protein structure prediction programs have been used to propose preliminary structural models based on the corresponding DNA sequences (21, 42, 51). The sequencing data indicate that several yeast sugar transporters are homologous with bacterial and mammalian sugar transporters (5, 6, 42), such as several human glucose transporters (27). The members of this sugar transporter superfamily all have the same general structure, consisting of 12 potential membrane-spanning segments connected by hydrophilic loops of similar relative lengths (42). The structures of these transporters differ mainly in the lengths of their N termini and to a lesser extent in the lengths of their C-terminal segments. The human glucose transporters have very short N termini (12 amino acids long) (27), as do the bacterial transporters (24), while the yeast members of this family have N termini of 66 (42) to 97 (5) amino acids. All of the other yeast permeases sequenced so far (1,21,30,43,51) have similarly long N-terminal hydrophilic extensions. The orientation of these segments with respect to the plasma membrane has not been demonstrated experimentally for these yeast proteins. A number of yeast permeases carry potential N glycosylation sites, but there is as yet no clear indication as to whether any of these permeases is indeed a glycoprotein. These proteins have not yet been analyzed biochemically and no yeast permease has been purified, whereas several mammalian glucose transporters have been purified (19). Purification and extensive biochemical studies have been performed on the Escherichia coli lactose permease (11, 48), for which overexpression of active protein could be obtained (44). In contrast, several attempts at overexpression of active yeast transporters ...