Abstract
Sir2 is a central regulator of yeast aging and its deficiency increases daughter cell inheritance of stress- and aging-induced misfolded proteins deposited in aggregates and inclusion bodies. Here, by quantifying traits predicted to affect aggregate inheritance in a passive manner, we found that a passive diffusion model cannot explain Sir2-dependent failures in mother-biased segregation of either the small aggregates formed by the misfolded Huntingtin, Htt103Q, disease protein or heat-induced Hsp104-associated aggregates. Instead, we found that the genetic interaction network of SIR2 comprises specific essential genes required for mother-biased segregation including those encoding components of the actin cytoskeleton, the actin-associated myosin V motor protein Myo2, and the actin organization protein calmodulin, Cmd1. Co-staining with Hsp104-GFP demonstrated that misfolded Htt103Q is sequestered into small aggregates, akin to stress foci formed upon heat stress, that fail to coalesce into inclusion bodies. Importantly, these Htt103Q foci, as well as the ATPase-defective Hsp104Y662A-associated structures previously shown to be stable stress foci, co-localized with Cmd1 and Myo2-enriched structures and super-resolution 3-D microscopy demonstrated that they are associated with actin cables. Moreover, we found that Hsp42 is required for formation of heat-induced Hsp104Y662A foci but not Htt103Q foci suggesting that the routes employed for foci formation are not identical. In addition to genes involved in actin-dependent processes, SIR2-interactors required for asymmetrical inheritance of Htt103Q and heat-induced aggregates encode essential sec genes involved in ER-to-Golgi trafficking/ER homeostasis.
Highlights
Cell division in budding yeast, Saccharomyces cerevisiae, and specific adult stem/progenitor cells includes asymmetrical inheritance of oxidized proteins, ensuring low levels of cytosolic damage in a specific cell lineage [1,2,3]
We tested whether the passive diffusion model or the factor-dependent model appear most relevant in explaining asymmetrical inheritance by quantifying traits predicted to affect inheritance by passive diffusion and identifying factors required for asymmetrical inheritance amongst essential genes interacting with SIR2; a gene shown previously to be required for mother-biased segregation
We show that passive diffusion of aggregates is not sufficient to establish mother-biased segregation and that ER to Golgi trafficking, in addition to the actin cytoskeleton, calmodulin, and the Myo2 motor protein, are key components restricting the inheritance of both heat stressed-induced aggregates and aggregates formed of the Huntington disease protein Htt103Q
Summary
Cell division in budding yeast, Saccharomyces cerevisiae, and specific adult stem/progenitor cells includes asymmetrical inheritance of oxidized proteins, ensuring low levels of cytosolic damage in a specific cell lineage [1,2,3]. Besides the protein remodeling factor Hsp104, the yeast gerontogene Sir2 [9,10,11] is required for asymmetrical segregation of oxidized and aggregated proteins [1,4,12,13] The role of both Hsp104 and Sir in establishing damage asymmetry has been linked to actin cable-dependent processes and the polarisome [5,14]; a complex at the tip of the daughter cell required for actin cable nucleation [15,16].
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