How Fungi Sense Sugars, Alcohols, and Amino Acids
G-protein-coupled receptors (GPCRs) are an important receptor gene family and play important roles in sensing sugars in eukaryotic organisms, including fungi. Recent studies on the interactions between Cryptococcus and plants, a major environmental niche, reveal that myo-inositol, produced and secreted by plants, is sensed by the fungus and promotes fungal sexual reproduction, providing a potential explanation for how this organism completes its life cycle in nature. Several fusel alcohols, such as 1-butanol and isoamyl alcohol, stimulate filamentous growth of haploid cells. A recent study showed that aromatic alcohols (such as tryptophol and phenylethanol) secreted by yeast cells function as quorum-sensing molecules and stimulate filamentous growth through a Flo11-dependent mechanism. Amino acids are important nutrients for fungi and are detected by specialized sensor systems, which include the general amino acid permease Gap1, the Ssy1-Ptr3-Ssy5 (SPS) system, GPCRs, and the target of rapamycin (TOR). In Neurospora crassa, three transport systems have been described based on the analysis of the kinetics of amino acid uptake and the patterns of competitive inhibition between amino acids. The Gap1 homolog in Neurospora crassa is encoded by the PMG locus, which can transport all L-amino acids except proline. Besides this system, two other transport systems have also been identified. One is encoded by the MTR gene and transports neutral and aromatic amino acids. The other is encoded by the PMB gene and transports basic amino acids, such as arginine and lysine.
