Davis, Jonthan , Liepman, Aaron H. , Keegstra, Kenneth .
Topology of Two Golgi-Membrane Glycan Synthases.
Xyloglucan and galactomannan are important hemicellulosic polysaccharides in plant cell walls. The enzymes responsible for synthesizing the glycan backbones of these polysaccharides were recently identified. These enzymes belong to a large group called cellulose synthase-like (CSL) proteins, which may be further divided into families (A-H) based on sequence similarity. Several CSLA proteins synthesize the mannan backbone of galactomannan, while selected CSLCs can synthesize the glucan backbone of xyloglucan. (Liepman et al., (2005) PNAS 102:2221, Cocuron et al., (2007) PNAS, in press) Hemicelluloses are produced in the Golgi by polytopic glycan synthases, while cellulose synthesis by CESA occurs at the plasma membrane. CESA proteins deposit cellulose microfibrils into the extracellular matrix, transferring glucose from cytosolic UDP-glucose to the elongating polymers. Polymerization by CSLs may be coupled with intrinsic translocase activity similarly to CESA, or glycan synthesis in the Golgi might be dependant on sugar-nucleotide transporters. Transmembrane domain (TMD) prediction programs suggest that CSLAs contain five TMDs, while CSLCs are predicted to have six. To examine CSL membrane topology, Arabidopsis CSLA9 and CSLC4 were expressed Pichia pastoris with terminal T7 epitope fusions. Susceptibility of the epitopes in intact microsomes to protease cleavage was used to indicate orientation. Data from these experiments, and the predicted positions of the TMDs, suggest that the catalytic domain of CSLA9 is in the lumen, whereas the catalytic domain of CSLC4 is predicted to be cytosolic. This presents the possibility that CSLCs may function similarly to CESAs, while CSLAs might depend on a GDP-mannose transporter to deliver their substrate to the catalytic domain.
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1 - Michigan State University, DOE Plant Research Lab, 110 Plant Biology Building, 178 Wilson Rd., East Lansing, MI, 48824, USA
2 - Eastern Michigan, Biology Department
3 - Michigan State University, DOE Plant Research Lab
Presentation Type: Plant Biology Abstract
Location: Exhibit Hall (Northeast, Southwest & Southeast)/Hilton
Date: Sunday, July 8th, 2007
Time: 8:00 AM