Writing the catalog of plasma membrane repair proteins

(Press-News.org) In the evolutionary history of life, the ability of a cell to separate its inner world from the external environment was an important turning point. The so-called plasma membrane lets cells control what gets in and out and allows them to communicate and cooperate with one another, creating the conditions for complex, multicellular life.

This barrier is fragile. Every day, mechanical stress, environmental changes, and bacterial toxins threaten to puncture the membrane, and if the wounds aren’t sealed and healed quickly, the cell dies. Despite its importance to the survival of our cells, the processes of plasma membrane repair have remained elusive. Their relevance is underscored by the fact that mutations in plasma membrane repair proteins cause various diseases, likely arising from cell death due to unrepaired plasma membrane.

But now, researchers at the Okinawa Institute of Science and Technology (OIST) have captured this repair mechanism in new detail. Using budding yeast as a model organism, they identified 80 proteins involved in plasma membrane repair, 72 of them never reported before, and tracked their movements in real time. Their findings are published in eLife. “This is the first large-scale catalog of plasma membrane repair proteins, and a timeline of how the process unfolds,” says lead author Dr. Yuta Yamazaki of the Membranology Unit at OIST.

The team combined proteome-wide screening with advanced live-cell imaging. First, they scanned thousands of yeast proteins under normal and plasma membrane stress conditions. Then, they used laser-induced damage to puncture single cells and tracked protein movements over time.

From this, they discovered a coordinated sequence of molecular events. The first responders were proteins from the Pkc1 signaling pathway. Next came exocytosis, a process where vesicles inside the cell fuse with the plasma membrane to deliver fresh lipids and structural components, essential building blocks for sealing the wound. Then, they observed clathrin-mediated endocytosis (CME), which folds the plasma membrane inwards to form a pocket to transport lipids and membrane proteins from the external environment into the cell. CME likely helps ensure that the repaired membrane regains its normal structure and function.

While the involvement of Pkc1 and exocytosis was expected, the role of CME in the repair process in budding yeast was a surprise. “Endocytosis at the damage site was previously reported in mammalian cells, but not in budding yeast,” explains Dr. Yamazaki. “We show it happens in yeast too, suggesting that this is an ancient repair mechanism from before mammals and yeast differentiated.”

Another important finding was that many proteins normally located at the growing bud tip, where new membrane is built, abandoned their usual position and moved to the damage site. “The same proteins that create new membranes stop working at the bud and come to repair the damage,” says Dr. Yamazaki. “The machinery of wound repair appears be similar as the one involved in making new membrane.”

Dr. Yamazaki concludes: “Our dataset provides a foundation for researchers to investigate plasma membrane repair mechanisms in higher eukaryotes, including human cells.” Given the link between plasma membrane damage and cellular (and thereby organismal) aging previously discovered by the group, and given that defects in the membrane repair processes are linked to diseases including muscular dystrophy, fundamental research of these ancient mechanisms common across cellular life helps establish the necessary baseline for future therapeutic application – as well as for investigations into the evolutionary origins of cells and the core mechanisms of cellular life.  

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