Asymmetric inheritance of the yeast chaperone Hsp26p and its functional consequences

The yeast Hsp26 protein, a conserved a-crystallin small heatshock chaperone, is assembled in to oligomeric complexes, microscopically visible as distinct cytoplasmic foci. We studied at single cell resolution the dynamics of Hsp26p foci assembly, the mode of their inheritance in to progeny cells and the physiological significance of Hsp26p function. We showed that Hsp26p foci are formed upon cells' entry in to stationary phase, but upon re-entry to proliferation they are asymmetrically retained in the mother cells and are absent from the newborn daughters. Despite the fact that Hsp26p assists re-solubilization of aggregation-prone proteins it does not extend chronological life span nor does it increase the tolerance of either mother or daughters against lethal stresses. Upon sequential HSP26 inductions, newly synthesized Hsp26p is readily incorporated in pre-existing foci, generating larger in size, but similar in appearance foci. At extreme heat-shock conditions, Hsp26p foci break apart into smaller granules dispersed in both mothers and growing buds, while recovery at normal temperature results in Hsp26p foci reassembly. These results suggested that such a complicated mechanism of dynamic Hsp26p assembly and disassembly, as well as asymmetric segregation may contribute to fine tuning regulation of protein aggregates' refolding, cell fitness and survival.