Academic journal article Genetics

Genome Analysis of the Unicellular Green Alga Chlamydomonas Reinhardtii Indicates an Ancient Evolutionary Origin for Key Pattern Recognition and Cell-Signaling Protein Families

Academic journal article Genetics

Genome Analysis of the Unicellular Green Alga Chlamydomonas Reinhardtii Indicates an Ancient Evolutionary Origin for Key Pattern Recognition and Cell-Signaling Protein Families

Article excerpt

ABSTRACT

The evolution of specific cell signaling and adhesion domains may have played an important role in the transition to a multicellular existence in the metazoans. Genomic analysis indicates that several signaling domains predominately found in animals are also present in the unicellular green alga, Chlamydomonas reinhardtii. A large group of proteins is present, containing scavenger receptor cysteine-rich (SRCR) and C-type lectin domains, which function in ligand binding and play key roles in the innate immune system of animals. Chlamydomonas also contains a large family of putative tyrosine kinases, suggesting an important role for phosphotyrosine signaling in the green algae. These important signaling domains may therefore be widespread among eukaryotes and most probably evolved in ancestral eukaryotes before the divergence of the Opisthokonts (the animal and fungal lineage).

THE evolution of multicellularity requires the development of complex signaling mechanisms to regulate intercellular interactions such as tissue differentiation and pathogen defense. Comparative genomics approaches suggest that certain cell-signaling and adhesion protein domains are unique to animals (Arabidopsis Genome Initiative 2000; Hynes and Zhao 2000; Rubin et al. 2000). Many of these protein domains may have evolved as a result of specialization within multicellular animals, although others may have been present early in a unicellular ancestor of the metazoans and contributed to primitive cell-cell interactions. As a result,much interest has focusedonthe cell-signaling and adhesion proteins present in the unicellular relatives of the metazoa, the choanoflagellates (King et al. 2003, 2008; Steenkamp et al. 2006).

The completion of the genome of the biflagellated chlorophyte alga, Chlamydomonas reinhardtii, represents an important addition to the array of sequenced eukaryotes (Merchant et al. 2007).We present an analysis of the Chlamydomonas genome, indicating that many proteins are present that contain scavenger receptor cysteine-rich (SRCR) domains, C-type lectin domains, and putative tyrosine kinase domains. These protein domains are largely associated with the metazoa (Muller 2001). Chlamydomonas is a member of the chlorophyte algae, which diverged from the land plants >1000 million years ago (MYA), following the earlier divergence of the Opisthokonts, containing the animal and fungal lineages (1600 MYA) (Yoon et al. 2004). Genes shared between chlorophytes and animals were presumably present in ancestral eukaryotes, indicating a truly ancient origin for these cellular processes.

SRCR and C-type lectins: The SRCR and C-type lectin domains are both important in the recognition of pathogen-associated molecular patterns (PAMPs) by the metazoan innate immune system (Gordon 2002). The SRCR domain is a highly conserved cysteine-rich domain, first defined during the analysis of the type I macrophage scavenger receptor (Freeman et al. 1990), but subsequently identified in many different proteins with diverse functions and domain organizations. The sea urchin, Strongylocentrotus purpuratus, contains an extraordinarily large gene family of SRCR-domaincontaining proteins, many of which may function in the immune response (Pancer 2000; Rast et al. 2006). SRCR proteins perform a variety of other cellular functions, including the well-characterized speract receptor in the flagella of sea urchin sperm that mediates chemotactic responses to the egg (Dangott et al. 1989; Cardullo et al. 1994; Resnick et al. 1994). The C-type lectins were initially defined as animal-type lectins displaying Ca21-dependent carbohydrate-binding activity (Drickamer 1988), although the C-type lectin domain (CTLD) has since been identified in many diverse proteins, many of which may not bind carbohydrate or Ca21 (Zelensky and Gready 2005). CTLD proteins such as the mannose receptor (MR) and Dectin-1 have a well-characterized role in the mammalian innate immune systems (Brown andGordon 2001; East and Isacke 2002; McGreal et al. …

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