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Different types of cellular membranes have unique lipid compositions that are important for their functional identity. PI(4,5)P2 is enriched in the plasma membrane where it contributes to local activation of key cellular events, including actomyosin contraction and cytokinesis. However, how cells prevent PI(4,5)P2 from accumulating in intracellular membrane compartments, despite constant intermixing and exchange of lipid membranes, is poorly understood. Using the C. elegans early embryo as our model system, we show that the evolutionarily conserved lipid transfer proteins, PDZD-8 and TEX-2, act together with the PI(4,5)P2 phosphatases, OCRL-1 and UNC-26/synaptojanin, to prevent the build-up of PI(4,5)P2 on endosomal membranes. In the absence of these four proteins, large amounts of PI(4,5)P2 accumulate on endosomes, leading to embryonic lethality due to ectopic recruitment of proteins involved in actomyosin contractility. PDZD-8 localizes to the endoplasmic reticulum and regulates endosomal PI(4,5)P2 levels via its lipid harboring SMP domain. Accumulation of PI(4,5)P2 on endosomes is accompanied by impairment of their degradative capacity. Thus, cells use multiple redundant systems to maintain endosomal PI(4,5)P2 homeostasis.

Chapter 2. Systematic knock-out of SMP proteins in C. elegans reveals redundant functions of PDZD-8 and TEX-2 in maintaining endosomal PI(4,5)P2 homeostasis during early embryogenesis 19 Chapter 3. PDZD-8 and TEX-2 act in parallel with PI(4,5)P2 phosphatases to promote robust embryogenesis via regulation of endosomal PI(4,5)P2 homeostasis 39 Chapter 4. Asymmetric localization of ESYT-2 in C. elegans early embryos is regulated by PAR polarity determinant proteins 78 Chapter 5. Conclusion and future directions 97