Strong growth polarity of yeast prion fiber revealed by single fiber imaging

Using the yeast prion as a model, we have developed a novel system to obser ve the growth of individual prion fibers directly. NM fragments, the prion- determining region of the yeast protein Sup35p, were labeled by either red or green fluorescent dyes, and the fiber growth was observed under a fluore scence microscope. When green-Sup35NM was added to the preformed fibers mad e of red-Sup35NM, 70-97% of green fibers grew unidirectionally, from only o ne end of individual red fibers, whereas the remainder grew from both ends. Similarly, the majority of red fibers grew from only one end of green fibe rs when the order of addition was reversed. Sonication of preformed fibers to expose fresh ends did not change the results, excluding a possibility of occasional deformation of one end as the reason of the apparent unidirecti onal growth. These results indicate the polarity of Sup35 prion fibers and impose constraints on the models of fiber growth.