- Mar 11, 2019
Date & Location: March 11, 2019, at 4p; Room 101 Biochemistry Building
Subject: Elucidation of the xyloglycan-synthesizing multiprotein complexes in Golgi and how they can determine the polysaccharide native patterns
Host: Federica Brandizzi
About the Speaker
University: Iowa State University
Abstract: Xyloglucan is the major hemicellulosic polysaccharide in the primary cell walls of most flowering plants. It has multiple important functions in cell wall structural organization and plant growth regulation. The complex xyloglucan structure is synthesized in plant Golgi by several highly specific glycosyltransferases (XXT1, XXT2, XXT5, MUR3, XLT2 and FUT1) and glucan synthase (CSLC4). This knowledge of critical glycosyltransferases obtained through reverse-genetics and biochemical approaches makes the xyloglucan a good model for studies of polysaccharide biosynthesis in Golgi. Using cell biology and immunoprecipitation approaches we have demonstrated that xyloglucan is synthesized by the multiprotein complex, where all glycosyltransferases are, most likely, organized around the glucan synthase. Three xyloglucan xylosyltansferases (XXT1, XXT2, and XXT5) are essential for the initial xylosylation of the glucan backbone. Recently, we solved the 3D structure of XXT1 using high-resolution X-ray crystallography and constructed homology models of XXT2 and XXT5. Results from computational docking suggest subunit interfaces of the homodimer XXT1 and the heterodimer XXT2-XXT5 are similar; however, different surfaces of the XXT1 homodimer and the XXT2 subunit in the XXT2- XXT5 heterodimer can interact to form a linear trimer of dimers in which the XXT1 homodimer occupies the central position, thus confirming our experimental observations. All results obtained in the lab allowed us to propose the reasonable model of how glucan synthase CSLC4 and three XXTs are organized in a single complex to synthesize the native patterns of plant xyloglucan.
Using cell biology approches, we also continue to investigate how glycosyltransferases are transported from ER to Golgi to reveal requirements for their Golgi localization and protein-protein interactions. We have revealed that the short motifs on the protein cytosolic N-termini containing two argenin residues are required for Golgi localization of XXT2 and XXT5. We also demonstrated that transport of each XXT from ER to Golgi does not depend on the presence of the glucansynthase CSLC4 or other two XXTs.
The goal of our reaserch is to extend our limited undestanding of polysaccharide biosynthesis and mechanisms of glycosyltrasferase activities and substrate specificities.