Compartmentalization of ER-Bound Chaperone Confines Protein Deposit Formation to the Aging Yeast Cell

Abstract

In order to produce rejuvenated daughters, dividing budding yeast cells confine aging factors, including protein aggregates, to the aging mother cell. The asymmetric inheritance of these protein deposits is mediated by organelle and cytoskeletal attachment and by cell geometry. Yet it remains unclear how deposit formation is restricted to the aging lineage. Here, we show that selective membrane anchoring and the compartmentalization of the endoplasmic reticulum (ER) membrane confine protein deposit formation to aging cells during division. Supporting the idea that the age-dependent deposit forms through coalescence of smaller aggregates, two deposits rapidly merged when placed in the same cell by cell-cell fusion. The deposits localized to the ER membrane, primarily to the nuclear envelope (NE). Strikingly, weakening the diffusion barriers that separate the ER membrane into mother and bud compartments caused premature formation of deposits in the daughter cells. Detachment of the Hsp40 protein Ydj1 from the ER membrane elicited a similar phenotype, suggesting that the diffusion barriers and farnesylated Ydj1 functioned together to confine protein deposit formation to mother cells during division. Accordingly, fluorescence correlation spectroscopy measurements in dividing cells indicated that a slow-diffusing, possibly client-bound Ydj1 fraction was asymmetrically enriched in the mother compartment. This asymmetric distribution depended on Ydj1 farnesylation and intact diffusion barriers. Taking these findings together, we propose that ER-anchored Ydj1 binds deposit precursors and prevents them from spreading into daughter cells during division by subjecting them to the ER diffusion barriers. This ensures that the coalescence of precursors into a single deposit is restricted to the aging lineage.

Spatial regulation of coalesced protein assemblies: Lessons from yeast to diseases

“…Since the mechanisms ensuring proteostasis are not compromised during early aging, 15 we provided evidence that inclusions form through the collection of aggregate precursors which accumulate specifically in the mother cell over time. Somewhat similarly to other deposits, age-associated protein aggregates localize to the ER surfaces 16 . Taken together, the consensus from several aggregation models indicates that even tough they are membrane-less they localize and make use of endo-membranes to control their assembly and localization (Fig.…”

“…Ydj1 is anchored to ER membranes by being post-translationally farnesylated in its C-terminal CAAX box. 44 We found that this lipidation of Ydj1 was required for the retention of aggregate seeds in the mother cell 16 (Fig. 1B).…”

“…Since diffusion barriers are also found at the base of many cellular appendages and at the division plane of asymmetrically dividing metazoan cells, 48-50 this observation raises many interesting alternatives as to how cells might polarize their components so as to execute compartment-specific tasks. It is noteworthy that diffusion barriers and farnesylated Ydj1 are only confining aggregation precursors but not the large inclusions 16 . It is also interesting to note that, in addition to cytosolic aggregation precursors, misfolded proteins in the ER-lumen are also retained in the mother cell by diffusion barriers 46 .…”

“…During replicative aging the non-stressed cells, but not their rejuvenated daughter cells, form a single protein deposit 14,15 . Remarkably the single inclusions from 2 old cells rapidly merge to create a unique deposit after cell-cell fusion through mating 16 . Protein coalescence into a single deposit is not unique to single-celled organisms.…”

“…We identified the Hsp40 protein Ydj1 as a membrane anchor for aggregation precursors that were destined for the age-associated deposit 16 . Ydj1 is anchored to ER membranes by being post-translationally farnesylated in its C-terminal CAAX box.…”