Cell fusion is one of the most dramatic events a cell can undergo. Two membranes merge, two cytoplasms mingle, and two genomes may ultimately combine. In fungi, however, there is an added complication: before two cells can become one, each must breach its own protective armor. A new study from the University of Geneva (UNIGE), published in the journal Current Biology, reveals how yeast cells pull off this feat by deliberately blocking their own repair machinery at the fusion site — and how, when that block is removed, fusion can fail in spectacular fashion, with cells tearing themselves back apart after briefly merging.

Unlike animal cells, fungal cells are encased in a rigid cell wall, a structure that protects them from the environment and allows them to withstand the considerable internal pressure generated by their cellular contents. This wall is essential for survival, which is why it is monitored around the clock by damage-sensing systems that trigger repair the moment a breach appears. But during sexual reproduction, two fungal cells must do something that seems to contradict this survival logic: locally degrade the very wall that keeps them alive, so that their membranes can meet and merge. Once the wall is opened and the membranes fuse, the process normally proceeds until a single cell is formed, with the contents of both cells flowing freely through the new connection.

The apparent conflict between fusion and repair intrigued the team led by Sophie Martin, professor in the Section of Biology of the Faculty of Science at UNIGE. If the cell wall is constantly surveyed and patched, how do cells prevent their repair systems from undoing the very opening they need to complete reproduction? The answer, the new study shows, is that fusion is not simply a matter of opening a hole — it is an active campaign to keep that hole open by suppressing repair at exactly the right place and time.

To dissect this mechanism, the researchers turned to Schizosaccharomyces pombe, a single-celled yeast that has long served as a model organism for fundamental cellular processes, including the mechanics of sexual reproduction. In previous work, the team had identified mutant yeasts lacking an enzyme called Pak2. These mutants displayed what the researchers describe as transient fusion: the cells briefly merged and began exchanging cellular contents, only to separate again — an event with catastrophic consequences for the cells involved, which end up losing the contents they had started to share.

In normal cells, the team observed, Pak2 accumulates precisely at the site where the two cells make contact and fuse. Its activity promotes the opening and subsequent expansion of the fusion pore, the channel through which cellular contents pass from one cell to the other. In other words, Pak2 is not a general-purpose fusion factor scattered through the cell; it is deployed to a single, highly specific location where it drives the irreversible expansion of the connection between the two merging cells.

“However, when Pak2 is absent, the process can sometimes become reversible: the cells begin to fuse, exchange their contents, and then close the opening that connected them. In these cells, we observe that the mechanisms normally responsible for monitoring and repairing the cell wall become particularly active at the fusion site,” explains Julia Coronas-Serna, former postdoctoral researcher in Sophie Martin’s group and first author of the study. The observation suggests that Pak2’s normal role includes keeping the wall-repair machinery in check at the fusion interface; without it, the repair systems rush in and undo the connection.

Using advanced imaging techniques, the scientists examined the internal organization of cells lacking Pak2, focusing on the region where the two cells are in contact. What they saw was striking. “After the fusion pore closes in these mutant cells, we observe the formation of a new cell wall at the interface between the two cells. In other words, when the mechanisms that prevent repair are absent, the cell can literally rebuild the separation it had begun to dismantle,” says Sophie Martin. The failed fusion is thus not merely an aborted connection but an actively reconstructed boundary: the cells seal the pore with freshly built wall material, restoring the barrier they had opened moments before.

The findings reframe cell fusion as a tug-of-war between two opposing cellular programs. On one side is the fusion machinery, which locally degrades the wall and widens the pore; on the other is the surveillance and repair system, which senses wall damage and rebuilds it. Pak-family kinases such as Pak2 tip the balance decisively toward fusion by suppressing repair at the contact site, ensuring that once the pore opens and contents begin to flow, the process cannot be reversed. The irreversibility of fusion, long taken for granted as a simple consequence of membrane merging, now appears to be an actively enforced state maintained by a specific enzymatic signal.

This insight has implications well beyond yeast biology. The cell wall is a major target of many treatments against pathogenic fungi such as Candida and Aspergillus, which are responsible for serious infections in humans. Antifungal drugs frequently exploit the wall’s importance by interfering with its construction, but resistance and toxicity remain persistent challenges. By identifying the mechanisms that control the balance between cell wall repair and localized degradation during fusion, the UNIGE study opens new avenues in the search for antifungal strategies. A better understanding of how the wall’s formation and repair are regulated could ultimately help researchers identify new molecules capable of disrupting these processes in pathogenic species, potentially weakening fungal invaders at a structural level they cannot easily patch up.

More broadly, the study offers a vivid example of how cells coordinate seemingly incompatible processes — destruction and reconstruction — in space and time. The same wall that must be opened for reproduction must be protected for survival, and the cell resolves this paradox not by shutting down its repair systems globally, which would be lethal, but by locally overriding them at the fusion site through the action of a single enzyme. When that override fails, the consequences are immediate and dramatic: cells that had begun to merge rebuild the wall between them and retreat into separate lives, contents and all. Fusion, the study shows, is irreversible only as long as the cell actively keeps it that way.

Subject of Research: How the Pak2 enzyme blocks cell wall repair to make yeast cell fusion irreversible

Article Title: To fuse, cells block their repair system

Article References: To fuse, cells block their repair system. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: cell fusion, cell wall, yeast, Schizosaccharomyces pombe, Pak2 kinase, cell wall repair, sexual reproduction, antifungal strategies, Current Biology, University of Geneva, fungal pathogens, fusion pore