Incessant wear and tear on plasma membranes is a fact of cell life, so it's not surprising that cells know how to fix the resulting damage quickly—often within a few seconds. Norma Andrews and her colleagues (Yale University School of Medicine, New Haven, CT) have just revealed how plasma membrane repair works. In the process they have demonstrated a new function for the cell's decomposition specialists, the lysosomes.
Others have observed that membrane repair and calcium-regulated exocytosis often occur at the same time, with unidentified vesicles flocking to the site of injury. Andrews and her colleagues decided to test whether those vesicles were lysosomes, and if they were responsible for membrane repair. Via several methods—for example, using monoclonal antibodies against a major lysosomal glycoprotein—they conclude that blocking lysosomes' ability to fuse with the plasma membrane in a calcium- dependent manner also interferes with plasma membrane repair.
Andrews says she is now pondering how to investigate an intriguing idea: that lysosomes evolved not just to break down molecules inside cells, but to protect cells from invading microbes by pelting them with the resulting garbage. “We don't have any data indicating that, but it's an interesting speculation,” she says. “Paul McNeil suggested that plasma membrane repair may be a primitive form of secretion. We can imagine a scenario in which the involvement of lysosomes provided an additional evolutionary advantage. If the lysosomal contents were dumped on top of a microbe that was secreting membrane-damaging molecules close to a cell, this might create conditions outside the cell that would contain—or maybe even kill—the microbe.” ▪