Evolving views of pectin biosynthesis

Identification and characterization of a galacturonic acid transporter from Neurospora crassa and its application for Saccharomyces cerevisiae fermentation processes

BACKGROUND

Pectin-rich agricultural wastes potentially represent favorable feedstocks for the sustainable production of alternative energy and bio-products. Their efficient utilization requires the conversion of all major constituent sugars. The current inability of the popular fermentation host Saccharomyces cerevisiae to metabolize the major pectic monosaccharide D-galacturonic acid (D-GalA) significantly hampers these efforts. While it has been reasoned that the optimization of cellular D-GalA uptake will be critical for the engineering of D-GalA utilization in yeast, no dedicated eukaryotic transport protein has been biochemically described. Here we report for the first time such a eukaryotic D-GalA transporter and characterize its functionality in S. cerevisiae.

RESULTS

We identified and characterized the D-GalA transporter GAT-1 out of a group of candidate genes obtained from co-expression analysis in N. crassa. The N. crassa Δgat-1 deletion strain is substantially affected in growth on pectic substrates, unable to take up D-GalA, and impaired in D-GalA-mediated signaling events. Moreover, expression of a gat-1 construct in yeast conferred the ability for strong high-affinity D-GalA accumulation rates, providing evidence for GAT-1 being a bona fide D-GalA transport protein. By recombinantly co-expressing D-galacturonate reductase or uronate dehydrogenase in yeast we furthermore demonstrated a transporter-dependent conversion of D-GalA towards more reduced (L-galactonate) or oxidized (meso-galactaric acid) downstream products, respectively, over a broad concentration range.

CONCLUSIONS

By utilizing the novel D-GalA transporter GAT-1 in S. cerevisiae we successfully generated a transporter-dependent uptake and catalysis system for D-GalA into two products with high potential for utilization as platform chemicals. Our data thereby provide a considerable first step towards a more complete utilization of biomass for biofuel and value-added chemicals production.

The cell biology of cellulose synthesis

Plant stature and development are governed by cell proliferation and directed cell growth. These parameters are determined largely by cell wall characteristics. Cellulose microfibrils, composed of hydrogen-bonded β-1,4 glucans, are key components for anisotropic growth in plants. Cellulose is synthesized by plasma membrane-localized cellulose synthase complexes. In higher plants, these complexes are assembled into hexameric rosettes in intracellular compartments and secreted to the plasma membrane. Here, the complexes typically track along cortical microtubules, which may guide cellulose synthesis, until the complexes are inactivated and/or internalized. Determining the regulatory aspects that control the behavior of cellulose synthase complexes is vital to understanding directed cell and plant growth and to tailoring cell wall content for industrial products, including paper, textiles, and fuel. In this review, we summarize and discuss cellulose synthesis and regulatory aspects of the cellulose synthase complex, focusing on Arabidopsis thaliana.

Plant cell wall engineering: applications in biofuel production and improved human health

Plant cell walls consist largely of cellulose, non-cellulosic polysaccharides and lignin. Concerted attempts are underway to convert wall polysaccharides from crop plant residues into renewable transport fuels and other valuable products, and to exploit the dietary benefits of cereal grain wall polysaccharides in human health. Attempts to improve plant performance for these applications have involved the manipulation of the levels and structures of wall components. Some successes in altering non-cellulosic polysaccharides has been achieved, but it would appear that drastic changes in cellulose are more difficult to engineer. Nevertheless, future prospects for both genetically modified (GM) and non-GM technologies to modify plant cell wall composition and structure remain bright, and will undoubtedly find applications beyond the current focus on human health and biofuel production.

Anti-cancer activities of pH- or heat-modified pectin

Despite enormous efforts that have been made in the search for novel drugs and treatments, cancer continues to be a major public health problem. Moreover, the emergence of resistance to cancer chemotherapy often prevents complete remission. Researchers have thus turned to natural products mainly from plant origin to circumvent resistance. Pectin and pH- or heat-modified pectin have demonstrated chemopreventive and antitumoral activities against some aggressive and recurrent cancers. The focus of this review is to describe how pectin and modified pectin display these activities and what are the possible underlying mechanisms. The failure of conventional chemotherapy to reduce mortality as well as serious side effects make natural products, such as pectin-derived products, ideal candidates for exerting synergism in combination with conventional anticancer drugs.

A three-stage model of Golgi structure and function

The Golgi apparatus contains multiple classes of cisternae that differ in structure, composition, and function, but there is no consensus about the number and definition of these classes. A useful way to classify Golgi cisternae is according to the trafficking pathways by which the cisternae import and export components. By this criterion, we propose that Golgi cisternae can be divided into three classes that correspond to functional stages of maturation. First, cisternae at the cisternal assembly stage receive COPII vesicles from the ER and recycle components to the ER in COPI vesicles. At this stage, new cisternae are generated. Second, cisternae at the carbohydrate synthesis stage exchange material with one another via COPI vesicles. At this stage, most of the glycosylation and polysaccharide synthesis reactions occur. Third, cisternae at the carrier formation stage produce clathrin-coated vesicles and exchange material with endosomes. At this stage, biosynthetic cargo proteins are packaged into various transport carriers, and the cisternae ultimately disassemble. Discrete transitions occur as a cisterna matures from one stage to the next. Within each stage, the structure and composition of a cisterna can evolve, but the trafficking pathways remain unchanged. This model offers a unified framework for understanding the properties of the Golgi in diverse organisms.