The asymmetry of proteins at the neck of budding yeast is dependent on the proper assembly of septins.
Lukasz Kozubowski, Jennifer Larson, Kelly Tatchell
Biochemistry and Mol. Biol., LSU Health Sciences Center, 1501 Kings Hwy., Shreveport, LA, 71130, USA
In the yeast S. cerevisiae, septins form a scaffold in the shape of a ring at the future budding site that rearranges into a collar at the mother-bud neck. Many proteins bind asymmetrically to the septin collar but the basis for this spatial restriction is unknown. We find that two septin-associated proteins, Bni4 and Kcc4, are asymmetrically localized on the septin ring before budding. Bni4 is located on the exterior of the septin ring and Kcc4 is located on the interior of the ring. At bud emergence, Bni4 stays associated with the mother side of the septin collar, whereas Kcc4 moves towards the bud and is bound to the daughter side of the collar. Unbudded cells treated with the actin polymerization inhibitor, Latrunculin A, often fail to form septin rings but assemble polarized septin caps. The asymmetry of Bni4 and Kcc4 is eliminated in these caps, indicating that the initial septin organization into a ring is actin-dependent and necessary for the asymmetry. The Bni4/Kcc4 asymmetry is largely retained in mutants (gin4, elm1, cla4 and cdc3-1) in which the septin collar structure is disrupted. This indicates that the asymmetry, once established before bud emergence, is difficult to eliminate in budded cells and is therefore a robust feature of the septin complex.
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