Abstract
Cell growth is tightly coupled to nutrient availability. The target of rapamycin (TOR) kinase transmits nutritional and environmental cues to the cellular growth machinery. TOR functions in two distinct multiprotein complexes, termed TOR complex 1 (TORC1) and TOR complex 2 (TORC2). While the structure and functions of TORC1 are highly conserved in all eukaryotes, including algae and plants, TORC2 core proteins seem to be missing in photosynthetic organisms. TORC1 controls cell growth by promoting anabolic processes, including protein synthesis and ribosome biogenesis, and inhibiting catabolic processes such as autophagy. Recent studies identified rapamycin-sensitive TORC1 signaling regulating cell growth, autophagy, lipid metabolism, and central metabolic pathways in the model unicellular green alga Chlamydomonas reinhardtii. The central role that microalgae play in global biomass production, together with the high biotechnological potential of these organisms in biofuel production, has drawn attention to the study of proteins that regulate cell growth such as the TOR kinase. In this review we discuss the recent progress on TOR signaling in algae.
Highlights
The amount and quality of nutrients regulate cell growth in all living organisms
Biochemical and genetic studies in yeast demonstrated that target of rapamycin (TOR) regulates these important cellular processes by two independent signaling branches defined by the TOR complex 1 (TORC1) and TOR complex 2 (TORC2) protein complexes [7,8,9,10]
This study showed that most algae have a single TOR protein, some species such as the marine alga Emiliania huxleyi encode three TOR kinases [14]
Summary
The amount and quality of nutrients regulate cell growth in all living organisms. In eukaryotes, the availability of nutrients is sensed through different signaling networks such as the TOR (target of rapamycin) kinase. Evolutionary studies have shown that core components of the TOR signaling network are conserved among divergent species and likely evolved from an archaic pathway of the last eukaryotic common ancestor [12]. The TOR kinase has been described in algae, indicating an early evolutionary origin of this signaling network in photosynthetic eukaryotes [13]. It has been reported that the growth of Chlamydomonas cells is inhibited by rapamycin [13], whereas early studies indicated that some plants are resistant to this drug [15]. The sensitivity of Chlamydomonas cells to rapamycin, together with the easy manipulation of this organism, its simple life cycle, and a growing array of genetic and molecular tools [18], have positioned this green alga as a convenient model system to investigate the TOR network in photosynthetic eukaryotes
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