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 (30°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.

Vacuolar protein sorting 1 (Vps1), the yeast homolog to human dynamin, is a GTP hydrolyzing protein, which plays an important role in protein sorting and targeting between the Golgi and late endosomal compartments. In this study, we assessed the functional significance of Vps1 in the membrane traffic towards the vacuole. We show here that vps1𝛥 cells accumulated FM4-64 to a greater extent than wild-type (WT) cells, suggesting slower endocytic degradation traffic toward the vacuole. In addition, we observed that two endosome-to-vacuole traffic markers, DsRed-FYVE and Ste2-GFP, were highly accumulated in Vps1-deficient cells, further supporting Vps1’s implication in efficient trafficking of endocytosed materials to the vacuole. Noteworthy, a simultaneous imaging analysis in conjunction with FM4-64 pulse-chase experiment further revealed that Vps1 plays a role in late endosome to the vacuole transport. Consistently, our subcellular localization analysis showed that Vps1 is present at the late endosome. The hyperaccumulation of endosomal intermediates in the vps1 mutant cells appears to be caused by the disruption of integrity of HOPS tethering complexes, manifested by mislocalization of Vps39 to the cytoplasm. Finally, we postulate that Vps1 functions together with the Endosomal Sorting Complex Required for Transport (ESCRT) complex at the late endosomal compartments, based on the observation that the double mutants, in which VPS1 along with singular ESCRT I, II and III genes have been disrupted, exhibited synthetic lethality. Together, we propose that Vps1 is required for correct and efficient trafficking from the late endosomal compartments to the vacuole.

Tor2 is an activator of the Rom2/Rho1 pathway that regulates 𝛼-factor internalization. Since the recruitment of endocytic proteins such as actin-binding proteins and the amphiphysins precedes the internalization of 𝛼-factor, we hypothesized that loss of Tor function leads to an alteration in the dynamics of the endocytic proteins. We report here that endocytic proteins, Abp1 and Rvs167, are less recruited to endocytic sites not only in tor2 but also tor1 mutants. Furthermore, we found that the endocytic proteins Rvs167 and Sjl2 are completely mistargeted to the cytoplasm in tor1𝛥tor2^ts double mutant cells. We also demonstrate here that the efficiency of endocytic internalization or scission in all tor mutants was drastically decreased. In agreement with the Sjl2 mislocalization, we found that in tor1𝛥tor2^ts double mutant cells, as well as other tor mutant cells, the overall PIP₂ level was dramatically increased. Finally, the cell wall chitin content in tor2^ts and tor1𝛥tor2^ts mutant cells was also significantly increased. Taken together, both functional Tor proteins, Tor1 and Tor2, are essentially required for proper endocytic protein dynamics at the early stage of endocytosis.

The elucidation of the organization and maintenance of the plasma membrane has been sought due to its numerous roles in cellular function. In the budding yeast Saccharomyces cerevisiae, a novel paradigm has begun to emerge in the understanding of the distribution of plasma membrane microdomains and how they are regulated. We aimed to investigate the dynamic interdependence between the protein complexes eisosome and TORC2, representing micro-domains MCC and MCT, respectively. In this study, we reveal that the eisosome organizer Pil1 colocalizes with the MCT marker Avo2. Furthermore, we provide evidence that the formation of MCT is dependent on both eisosome integrity and adequate levels of the plasma membrane phosphoinositide PI(4,5)P₂. Taken together, our findings indicate that TORC2, eisosomes, and PI(4,5)P₂ exist in an interconnected relationship, which supports the emerging model of the plasma membrane.

Intracellular trafficking is a field that has been intensively studied for years and yet there remains much to be learned. Part ofthe reason that there is so much obscurity remaining in this field is due to all the pathways and the stages that define cellulartrafficking. One of the major steps in cellular trafficking is fusion. Fusion is defined as the terminal step that occurs when acargo-laden vesicle arrives at the proper destination. There are two types of fusion within a cell: homotypic and heterotypicfusion. Homotypic fusion occurs when the two membranes merging together are of the same type such as vacuole tovacuole fusion. Heterotypic fusion occurs when the two membranes at play are of different types such as when anendosomal membrane fuses with a Golgi membrane. In this review, we will focus on all the protein components – Rabs,Golgins, Multisubunit tethers, GTPases, protein phosphatases and SNAREs – that have been known to function in both ofthese types of fusion. We hope to develop a model of how all of these constituents function together to achieve membranefusion. Membrane fusion is a biological process absolutely necessary for proper intracellular trafficking. Due to the degreeof importance multiple proteins are required for it to be properly carried through. Whether we are talking about heterotypicor homotypic fusion, any defects in the fusion machinery can result in disease states such as Parkinson’s and Alzheimer’sdisease. Although much research has significantly expanded our knowledge of fusion, there is still much more to belearned.