Zygote-specific protein with hydroxyproline-rich glycoprotein domains and lectin-like domains involved in the assembly of the cell wall of Chlamydomonas reinhardtii (Chlorophyta)

The cell wall of Chlamydomonas reinhardtii zygotes, which forms rapidly aft er the fusion of wall-free gametes, provides a tractable system for studyin g the properties and assembly of hydroxyproline-rich glycoproteins, the maj or proteinaceous components of green algal and plant cell walls. We report the cloning of the zsp2 gene and the analysis of its ZSP-2 product, a 58.9 kDa poly-peptide that is synthesized exclusively by zygotes, The protein co ntains two (SP), repeats, establishing it as a member of the cell wall hydr oxyproline-rich glycoproteins family. It also contains a 4-fold iteration o f an amino acid sequence centered around cysteine residues, a configuration found in both plant and animal lectins, Furthermore, we report four observ ations on pellicle composition and production, First, cell-free preparation s of the pellicle matrix are rich in hydroxyproline, arabinose, and galacto se and contain bundles of very long fibrils, Second, glutathione blocks pel licle formation and results in the accumulation of long fibrils in the grow th medium. Third, antibody to ZSP-8 also blocks pellicle formation, Fourth, ZSP-2 immunolocalizes to the boundary between the outer layers of the wall proper and the pellicle matrix. These observations are consistent with the possibility that the Cys-rich (glutathione-sensitive) lectin-like domains of ZSP-2 may bind to sugar residues on the long fibrils and anchor them to the cell wall, thereby initiating and maintaining pellicle formation.