Requirements of Slm proteins for proper eisosome organization, endocytic trafficking and recycling in the yeast Saccharomyces cerevisiae

Eisosomes are large immobile assemblies at the cortex of a cell under the membrane compartment of Can1 (MCC) in yeast. Slm1 has recently been identified as an MCC component that acts downstream of Mss4 in a pathway that regulates actin cytoskeleton organization in response to stress. In this study, we showed that inactivation of Slm proteins disrupts proper localization of the primary eisosome marker Pil1, providing evidence that Slm proteins play a role in eisosome organization. Furthermore, we found that slm (ts) mutant cells exhibit actin defects in both the ability to polarize cortical F-actin and the formation of cytoplasmic actin cables even at the permissive temperature (30A degrees C). We further demonstrated that the actin defect accounts for the slow traffic of FM4-64-labelled endosome in the cytoplasm, supporting the notion that intact actin is essential for endosome trafficking. However, our real-time microscopic analysis of Abp1-RFP revealed that the actin defect in slm (ts) cells was not accompanied by a noticeable defect in actin patch internalization during receptor-mediated endocytosis. In addition, we found that slm (ts) cells displayed impaired membrane recycling and that recycling occurred in an actin-independent manner. Our data provide evidence for the requirement of Slm proteins in eisosome organization and endosome trafficking and recycling.