Dynamics of Mitochondria and Actin Cytoskeleton Interactions in Budding Yeast

In the budding yeast, S. cerevisiae, mitochondrial distribution involves movements of the organelle from the mother cell to the bud (anterograde movement) and from the bud to the mother (retrograde movement). Both types of movements require actin cables, dynamic bundles of F-actin filaments, which undergo continuous retrograde flow from the bud into the mother and provide tracks for mitochondrial movement. Some of the molecular mechanisms of these movements have been revealed: (1) retrograde movement of mitochondria is characterized by passive travel on actin cables using the actin-binding action of the mitochore/ERMES complex; (2) anterograde movement of mitochondria against the retrograde flow of actin cables requires an actin polymerization-dependent mechanism that include the actions of Arp2/3 complex, Jsn1p and Aim7p. Much less is known about how the dynamic interactions at the interface between mitochondria and actin cytoskeleton affect the stability and maintenance of actin cables. Here we show that deletion of ARC15, a non-essential component of Arp2/3 complex, results in decreased actin cable abundance. Deletion of AIM7 causes a decrease in retrograde actin cable flow. We also show that mitochondrial mass can influence the maintenance of actin cables. Cells grown under conditions where mitochondrial mass is modulated show different levels of actin cable thickness: there is an increase in actin cable thickness in cells with higher mitochondrial mass. This observation suggests that not only does the actin cytoskeleton affect mitochondrial distribution, but reciprocally, mitochondrial distribution can also affect the actin cytoskeleton. In case of cells with more mitochondria, changes in actin cable thickness seem to be essential to meet the demands of increased mitochondrial mass.