Yeast Thigmotropism Controlled by a Cell-Surface Mucin and a MAP-Kinase Pathway

Budding yeast can switch from growth as ovoid cells to growth in an adhesive and invasive filamentous form. Limiting availability of reduced nitrogen is thought to be the signal that induces filamentous growth. In contrast, we find that filamentation is a thigmotropic response. In the absence of repressive agents, growth on a surface is necessary and sufficient to induce filamentation, whereas nitrogen limitation is neither sufficient nor necessary. The thigmostimulus is transmitted to the genome via a MAP-kinase pathway that has been linked to filamentation. The FLO11 gene, encoding a cell-surface mucin protein, maps genetically at the top and at the bottom of this thigmotropism MAP-kinase pathway. These results suggest that yeast differentiation to the filamentous form is primarily a response to surface contact for the purposes of colonization and invasion, rather than foraging for nutrients.出芽酵母（budding yeast）可从卵圆形细胞的生长模式，转变为兼具黏附性与侵袭性的丝状生长模式。此前学界普遍认为，还原氮素的可及性不足是诱导丝状生长的信号。而本研究发现，丝状生长实则属于向触性（thigmotropic）应答：在不存在抑制性因子的条件下，于固体表面生长既是诱导丝状生长的必要条件，也是充分条件；而氮素限制既非该过程的充分条件，亦非必要条件。该向触性刺激通过一条与丝状生长相关的丝裂原活化蛋白激酶（MAP-kinase）通路传递至基因组。编码细胞表面黏蛋白的FLO11基因，在遗传定位上处于该向触性丝裂原活化蛋白激酶通路的上游与下游区域。上述结果表明，酵母向丝状形态的分化本质上是对表面接触的应答，以实现定植与侵袭，而非为了搜寻营养物质。