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Sridhar, Vaishnavi

University of British Columbia

Eukaryotic organelle membranes are composed of lipids and proteins. Most lipid precursors are synthesized in the endoplasmic reticulum (ER) and are transported to other membranes. Transport can proceed either via the vesicular or the non-vesicular route, primarily via lipid transport proteins at membrane contact sites (MCSs). MCSs are sites of close apposition between organelle membranes, which are present in eukaryotes including plants, fungi and animals. These sites are maintained by protein tethers, which function in lipid transport, maintenance of membrane integrity, membrane biogenesis and ion regulation. One such protein tether, Vps13, is highly conserved and functions as a bulk lipid transporter at many MCSs. Bulk transport of lipids is required to confer rapid changes in lipid composition or for membrane elongation, such as in case of generation of prospore membranes or for autophagosome formation or during endocytosis. At ER-PM MCSs, Vps13-like proteins could function in membrane elongation during endocytosis, thus making it important to identify Vps13-like proteins at these sites. I identified an uncharacterized protein Fmp27 and its paralog Ypr117w, which have secondary structure similarities to Vps13, with a potential role in lipid homeostasis at ER-PM MCSs. Fmp27 is a well conserved protein, and its mammalian ortholog is overexpressed in breast cancer, whereas its fly and plant orthologs are implicated in organismal development. I found that Fmp27 is anchored to the ER and localizes to ER-PM cortical sites. Furthermore, I identified an interacting partner, Ybl086c, which binds to Fmp27 and recruits it to these cortical sites via a conserved region in its C-terminus. I found that Fmp27/Ypr117w and Ybl086c are not required for the growth of cells in response to various stresses such as temperature, salt, osmolarity and detergents. Using fluorescent lipid probes, I found that these proteins do not have a clear role in the distribution of the glycerophospholipids phosphatidylserine (PS), phosphatidylinositol-4-phosphate (PI4P) and phosphatidylinositol-4,5-bisphosphate PI(4,5)P₂ in cells. Instead, I found that the deletion of Fmp27/Ypr117w and its binding partner confer sensitivity to the drug amphotericin B, indicating elevated levels of accessible sterols at the plasma membrane. This indicates a role for these proteins in lipid homeostasis.

Text

2025-04-30

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/84436

Cell and Developmental Biology

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/