Polarity studies in fission yeast have focused on the tea1p protein. It can be seen hitching a ride on growing microtubules as they speed toward the two ends of the cell—the only sites where growth takes place in fission yeast. Now, Snaith and Sawin have found a protein called mod5p that is localized to cell ends and helps to keep the arriving tea1p anchored to those same sites. Cells lacking mod5p delivered tea1p as usual but failed to keep it localized at the cell ends. Mod5p, in turn, was found all around the plasma membrane when tea1p was no longer present. Thus, the authors believe that tea1p and mod5p feedback on each other to ensure immobility.
There is preliminary evidence to suggest an indirect physical link between tea1p and mod5p. This linkage may cement both proteins in a complex that is big enough or sticky enough to be inherently immobile, thus anchoring them near the site where tea1p arrives. Alternatively, the association of the two proteins may trigger a biochemical change in the complex that fixes the complex in place. Either way, Sawin believes that the next step will be more biochemistry to determine just what happens when tea1p is dropped off at the ends. ▪