Identification of candidate chimeric hydroxyproline-rich glycoprotein (HRGP) encoding genes in cell-wall less prasinophytes, Ostreococcus and Micromonas, using bioinformatics

Jae-Hyeok Lee1, Sabine Waffenschmidt2, Alexandra Worden3, and Ursula Goodenough1

1Washington University in St. Louis, MO 63130, U.S.A. 2UniversitŠt zu Kšln, Kšln, Germany 3Monterey Bay Aquarium Research Institute, CA 95039, U.S.A.

The cell walls of Chlamydomonas self-assemble from HRGPs distinguished by long runs of Pro interspersed with a subset of additional amino acids, where the sequences are usually organized as quasi-repetitive modules. The Hyp-rich domains adopt a polyproline-II secondary structure to generate extended fibers called shafts. Shaft domains are interspersed and/or terminate with globular domains; hence these HRGPs are designated chimeric. The cell walls of land plants, while dominated by polysacchrides like cellulose, can be up to 10% HRGP, and a subpopulation of this land-plant HRGP family is also chimeric. Whereas prasinophytes usually have scale-based walls, Ostreococcus and Micromonas apparently lack any walls, but nonetheless carry candidate HRGP-encoding genes, as initially identified by their Pro-rich character. Forty eight and 124 genes were identified in the O. tauri and M. pusilla RCC299 genomes respectively, as compared with 209 and 238 genes for C. reinhardtii and A. thaliana respectively (and with only 8 unusual versions for the rhodophyte C. merolae). All the prasinophyte genes are predicted to encode chimeric proteins, and their shaft repeat modules (e.g. SP2-SP6, XP3) are also found in Chlamydomonas and A. thaliana, indicating a deep origin for HRGPs after red-algal divergence. The rich endowment of putative cell-wall HRGP genes in these cell wall-less prasinophytes raises the possibility that a subset associates with the cell surface without forming a distinct wall, a putative function being to stabilize interactions with cyanobacterial symbionts. Another possibility is that these "wall-less" picoeukaryotes in fact produce an HRGP-rich wall at some non-vegetative stage in their life cycle.

e-mail address of presenting author: jleec@artsci.wustl.edu