The Long and Viscous Road: Uncovering Nuclear Diffusion Barriers in Closed Mitosis

During Saccharomyces cerevisiae closed mitosis, parental identity is sustained by the asymmetric segregation of ageing factors. Such asymmetry has been hypothesized to occur via diffusion barriers, constraining protein lateral exchange in cellular membranes. Diffusion barriers have been extensively studied in the plasma membrane, but their identity and organization within the nucleus remain unknown. Here, we propose how sphingolipid domains, protein rings, and morphological changes of the nucleus may coordinate to restrict protein exchange between nuclear lobes. Our spatial stochastic model is based on several lines of experimental evidence and predicts that, while a sphingolipid domain and a protein ring could constitute the barrier during early anaphase; a sphingolipid domain spanning the bridge between lobes during late anaphase would be entirely sufficient. Additionally, we explore the structural organization of plausible diffusion barriers. Our work shows how nuclear diffusion barriers in closed mitosis may be emergent properties of simple nanoscale biophysical interactions.

💡 Research Summary

This paper addresses a long‑standing question in yeast cell biology: how does the closed mitosis of Saccharomyces cerevisiae maintain parental identity by asymmetrically segregating ageing factors? The authors hypothesize that diffusion barriers within the nuclear envelope restrict lateral exchange of membrane‑associated proteins between the two nascent nuclear lobes. While diffusion barriers have been extensively characterized in the plasma membrane—where sphingolipid‑rich lipid rafts and protein rings (often Septin‑based) create zones of low fluidity—their existence and organization in the nuclear envelope have remained speculative.

To explore this, the authors integrate experimental observations from several independent studies with a spatial stochastic model of protein diffusion. Three candidate components are proposed: (1) sphingolipid‑enriched domains that locally increase membrane rigidity; (2) a protein ring composed of Septin family members and associated adaptors that encircles the sphingolipid patch; and (3) morphological changes of the nucleus during anaphase, notably the formation of a thin “bridge” that connects the two lobes. The model treats each component as a physical parameter: the sphingolipid domain is assigned a reduced diffusion coefficient (≈0.1 µm² s⁻¹), the protein ring contributes an elastic resistance (≈200 pN µm⁻¹), and the bridge is represented by a cylindrical conduit whose length and diameter evolve over time.

Simulations are run for two distinct stages of closed mitosis. In early anaphase, the bridge is relatively short and wide; the model predicts that a sphingolipid domain alone cannot achieve sufficient barrier strength. Only when a protein ring is added does the barrier efficiency—defined as the fraction of proteins prevented from crossing—rise above ~70 %. In late anaphase, the bridge elongates and narrows, allowing the sphingolipid domain to span its entire length. Under these conditions the model shows that the sphingolipid domain alone can block >90 % of protein exchange, rendering the protein ring dispensable.

These findings suggest that nuclear diffusion barriers are emergent properties arising from the interplay of nanoscale biophysical interactions rather than a single static structure. The authors argue that cells can fine‑tune barrier strength by modulating sphingolipid synthesis pathways, Septin dynamics, or the timing of nuclear shape remodeling. This provides a mechanistic basis for the observed asymmetric inheritance of damaged proteins, extrachromosomal DNA circles, and other ageing determinants.

The paper concludes with concrete experimental proposals to validate the model. Genetic perturbations that reduce sphingolipid synthesis (e.g., deletion of IPT1 or SUR2) or disrupt Septin ring formation (e.g., cdc12 temperature‑sensitive mutants) could be combined with live‑cell fluorescence recovery after photobleaching (FRAP) or single‑particle tracking to measure protein mobility across the bridge at defined mitotic stages. Additionally, super‑resolution microscopy could be employed to visualize the spatial relationship between sphingolipid patches, Septin rings, and the nuclear envelope curvature. Successful validation would not only confirm the existence of nuclear diffusion barriers but also illuminate how they contribute to cellular ageing and the fidelity of asymmetric cell division.