Yeast chemotropism: A paradigm shift in chemical gradient sensing

Abstract

The ability of cells to direct their movement and growth in response to shallow chemical gradients is essential in the life cycles of all eukaryotic organisms. The signaling mechanisms underlying directional sensing in chemotactic cells have been well studied; however, relatively little is known about how chemotropic cells interpret chemical gradients. Recent studies of chemotropism in budding and fission yeast have revealed 2 quite different mechanisms-biased wandering of the polarity complex, and differential internalization of the receptor and G protein. Each of these mechanisms has been proposed to play a key role in decoding mating pheromone gradients. Here we explore how they may work together as 2 essential components of one gradient sensing machine. Cells polarize their structures and functions in response to a variety of directional stimuli, including gravity, 1 light, 2 electrical gradients, 3 and chemical gradients. 4,5 Cellular responses to chemical gradients have attracted the most attention, as they are essential to the life cycles of all eukaryotic species. The best-known gradient-stimulated cellular outputs, chemotaxis (directed cell movement), and chemotropism (directed cell growth), are required for a wide range of biologic processes. For example, chemotaxis plays a vital role in development, immunity, wound healing, inflammation, and metastasis; chemotropism is integral to axon guidance, 6,7 angiogenesis, 8-10 pollen tube guidance, 11,12 and fungal life cycles. 13,14 Although they ultimately exhibit quite different behavior, chemotactic and chemotropic cells face similar challenges. In both cases, the responding cell must sense small differences in chemical concentration across its surface, determine the direction of the gradient source, and then polarize its cytoskeleton toward or away from it. Whereas a great deal has been learned about the mechanisms underlying directional sensing in chemotactic models, 15,16 relatively little is known about how chemotropic cells interpret chemical gradients. To date, the best-characterized chemotropic model is the mating response of the budding yeast, Saccharomyces cerevisiae. Haploid yeast exist as 2 mating types, MATa and MATa, each of which secretes a peptide pheromone that binds to a G protein-coupled receptor (GPCR) on cells of the opposite type. Ligand-bound receptor induces the activation of Ga and consequent dissociation of the heterotrimeric G protein into its Ga and Gbg subunits. Free Gbg then activates a MAPK cascade that triggers cell-cycle arrest in G1 and changes in gene expression. Once arrested in G1, pheromone-stimulated cells also form mating projections via actin-cable-directed delivery of membrane vesicles to a focused growth site (for reviews, see references 17,18). Local activation of the conserved Rho-family GTPase Cdc42 by its guanine nucleotide exchange factor Cdc24 determines the position of actin-cable nucleation, and thus the direction of mating projection growth. 19 Cdc24 and Cdc42 are members of a group of interacting proteins collectively known as the polarity complex. When pheromone-stimulated cells are unable to sense a gradient (e.g., under isotropic or saturating dose conditions), they form mating projections where they would have budded in the next cell cycle. 20 This position, marked by Bud1, is known as the default polarity site. 21 In mating mixtures, however, proxi-mal cells of opposite mating type orient their mating projections directly toward one another. The projections continue to elongate and narrow until they touch at their tips and fuse. 22 Although early observations of yeast mating were suggestive of chemotropism, the first clear demonstration that yeast can align their growth to a gradient was reported by Segall (1993), 23 who showed that mating projections grow toward a point source of synthetic pheromone, and that they reorient (track the gradient) when the point source is moved. Segall 23 also concluded that a 1% gradient across the diameter of the yeast cell is steep enough to elicit an accurate directional response. Genetic studies in which MATa cells were challenged to discriminate between pheromone-secreting and a large excess of non-secreting partners confirmed that yeast mating is gradient-driven and of remarkably high fidelity. 24,25 More recently, gradient-stimulation of yeast in microfluidic devices confirmed Segall's observations, 26,27 and time-lapse imaging of mating mixtures has provided definitive examples of yeast chemotropism in situ. 28 How do yeast cells accurately sense the direction of the pheromone source and polarize their growth toward it? The discovery that Gbg interacts with Cdc24 via the Far1 scaffold protein 29,30 suggested that Gbg links activated receptors to the actin-nucleation machinery, and, in so doing, acts as