Identification and characterization of hydroxyproline-rich glycoprotein networks for building cell walls in the green lineage
Jae-Hyeok Lee and Ursula Goodenough
Dept. Biology, Washington University, 1 Brookings Dr., Saint Louis, MO 63130, USA
Land plant cell walls are dominated by polysaccharides like cellulose and pectin, but also include hydroxyproline-rich glycoproteins (HRGPs) that feature in green algal cell walls and are the sole components of Chlamydomonas reinhardtii cell walls. Because of their well-defined structure and assembly, Chlamydomonas cell walls are the best model to study how HRGPs self-assemble in building cell walls, and possibly to improve our understanding of HRGP roles in plant cell-wall formation. An algorithm has been developed to generate sets of HRGP-encoding candidates in algal and land plant genomes. The Chlamydomonas HRGP family with more than 600 members shows limited but meaningful homology to land plant HRGPs while displaying a spectacular radiation. Extensin and arabinogalactan protein (AGP) homologues were identified among the Chlamydomonas candidates, and their expression was analyzed to identify putative vegetative (V) wall and zygotic (Z) wall components. Based on the expected functions of extensins and AGPs in plants, models have been generated for V- and Z-wall assembly wherein 1) V-wall layers are sequentially assembled via globular domain-mediated delivery of individual components and 2) early Z-wall assembly is established via GPI-anchored HRGPs. Most Chlamydomonas HRGPs are chimeric, with fibrous and globular domains; the latter often display homologies with genes of known function, such as proteases, glycoside hydrolases, and lectins. Functional characterization of HRGPs is in progress by forward genetic screening for cell wall defects. We propose that HRGP divergence has been a foundational driver of morphogenetic diversity in the green lineage.
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