RESUMEN
Gradient-directed cell migration (chemotaxis) and growth (chemotropism) are processes that are essential to the development and life cycles of all species. Cells use surface receptors to sense the shallow chemical gradients that elicit chemotaxis and chemotropism. Slight asymmetries in receptor activation are amplified by downstream signaling systems, which ultimately induce dynamic reorganization of the cytoskeleton. During the mating response of budding yeast, a model chemotropic system, the pheromone receptors on the plasma membrane polarize to the side of the cell closest to the stimulus. Although receptor polarization occurs before and independently of actin cable-dependent delivery of vesicles to the plasma membrane (directed secretion), it requires receptor internalization. Phosphorylation of pheromone receptors by yeast casein kinase 1 or 2 (Yck1/2) stimulates their internalization. We showed that the pheromone-responsive Gßγ dimer promotes the polarization of the pheromone receptor by interacting with Yck1/2 and locally inhibiting receptor phosphorylation. We also found that receptor phosphorylation is essential for chemotropism, independently of its role in inducing receptor internalization. A mathematical model supports the idea that the interaction between Gßγ and Yck1/2 results in differential phosphorylation and internalization of the pheromone receptor and accounts for its polarization before the initiation of directed secretion.
Asunto(s)
Subunidades beta de la Proteína de Unión al GTP/metabolismo , Subunidades gamma de la Proteína de Unión al GTP/metabolismo , Receptores de Feromonas/metabolismo , Proteínas de Saccharomyces cerevisiae/metabolismo , Saccharomyces cerevisiae/metabolismo , Proteínas Adaptadoras Transductoras de Señales/genética , Proteínas Adaptadoras Transductoras de Señales/metabolismo , Algoritmos , Quinasa de la Caseína I/genética , Quinasa de la Caseína I/metabolismo , Membrana Celular/metabolismo , Polaridad Celular , Quimiotaxis , Simulación por Computador , Subunidades beta de la Proteína de Unión al GTP/química , Subunidades beta de la Proteína de Unión al GTP/genética , Subunidades gamma de la Proteína de Unión al GTP/química , Subunidades gamma de la Proteína de Unión al GTP/genética , Proteínas Activadoras de GTPasa/genética , Proteínas Activadoras de GTPasa/metabolismo , Proteínas Luminiscentes/genética , Proteínas Luminiscentes/metabolismo , Microscopía Confocal , Modelos Biológicos , Feromonas/metabolismo , Fosforilación , Unión Proteica , Multimerización de Proteína , Receptores del Factor de Conjugación/genética , Receptores del Factor de Conjugación/metabolismo , Receptores de Feromonas/genética , Saccharomyces cerevisiae/genética , Saccharomyces cerevisiae/crecimiento & desarrollo , Proteínas de Saccharomyces cerevisiae/química , Proteínas de Saccharomyces cerevisiae/genética , Transducción de Señal , Imagen de Lapso de Tiempo/métodosRESUMEN
Mating yeast cells interpret complex pheromone gradients and polarize their growth in the direction of the closest partner. Chemotropic growth depends on both the pheromone receptor and its associated G-protein. Upon activation by the receptor, Gα dissociates from Gßγ and Gß is subsequently phosphorylated. Free Gßγ signals to the nucleus via a MAPK cascade and recruits Far1-Cdc24 to the incipient growth site. It is not clear how the cell establishes and stabilizes the axis of polarity, but this process is thought to require local signal amplification via the Gßγ-Far1-Cdc24 chemotropic complex, as well as communication between this complex and the activated receptor. Here we show that a mutant form of Gß that cannot be phosphorylated confers defects in directional sensing and chemotropic growth. Our data suggest that phosphorylation of Gß plays a role in localized signal amplification and in the dynamic communication between the receptor and the chemotropic complex, which underlie growth site selection and maintenance.