Slow diffusion of proteins in the yeast plasma membrane allows polarity to be maintained by endocytic cycling

Many cells show a polarized distribution of some plasma membrane proteins, which may be maintained either by a diffusion barrier or kinetically: as first demonstrated in fibroblasts, locally exocytosed proteins will remain polarized if they are endocytosed and recycled before they can diffuse to equilibrium. In yeast, actin cables direct exocytosis to the bud and to the tips of polarized mating intermediates termed shmoos. A septin ring at the bud neck retains some proteins, but shmoos lack this. Here, we show that the exocytic SNARE Snc1 is kinetically polarized. It is concentrated at bud and shmoo tips, and this requires its endocytosis. Kinetic polarization is possible in these small cells because proteins diffuse much more slowly in the yeast plasma membrane than would be expected from measurements in animal cells. Slow diffusion requires neither the cell wall nor polymerized actin, but it is affected in the ergosterol synthesis mutant erg6. Other proteins also require endocytosis for efficient polarization, and the plasma membrane SNARE Sso1 can be polarized merely by appending an endocytic signal. Thus, despite their small size, yeast cells can use localized exocytosis and endocytic recycling as a simple mechanism to maintain polarity.