Polymers of the candidalysin peptide become membrane pores.

Abstract

C. albicans infection requires the action of the virulence factor candidalysin (CL), which damages the plasma membrane of target human cells (Moyes Nature 2016). However, the molecular mechanism that CL uses to permeabilize membranes was unknown. We have employed complementary biophysical, modeling, microscopy, and cell biology methods to reveal that CL forms membrane pores using a unique molecular mechanism (Russell eLife 2022). Unexpectedly, we observed that CL readily assembles into linear polymers in solution. We propose that the basic structural unit in polymer formation is a CL oligomer, which is sequentially added into a string configuration that can close into a loop. Our data indicate that CL loops spontaneously insert into the membrane to become membrane pores. We identified a CL mutation (G4W) that inhibited the formation of polymers in solution and prevented formation of pores in synthetic lipid membrane systems. Studies in epithelial cells showed that G4W CL failed to activate the danger response signaling pathway, a hallmark of the pathogenic effect of CL. These results indicate that CL polymerization in solution is a necessary step for the damage of cellular membranes. Analysis of thousands of CL pores by atomic force microscopy revealed co-existence of simple depressions and complex pores decorated with protrusions. Imaging and modeling indicate that the two types of pores are formed by CL oligomers assembled into alternate orientations. We propose that this structural rearrangement represents a maturation mechanism that stabilizes pore formation . The data show that CL uses a previously unknown mechanism to damage membranes, whereby pre-assembly of CL loops in solution directly leads to formation of membrane pores. Our investigation not only unravels a new paradigm for the formation of membrane pores, but additionally identifies CL polymerization as a novel therapeutic target to treat candidiasis.