“…Complementing this work, a recent study links ORP3 activation by PKC and Ca 2þ entry with a mechanism implicated in focal adhesions dynamics. Once localized at ER-PM contact sites, ORP3 interacts with IQSec1,a guanine nucleotide exchange factors of Arf5 to trigger focal adhesion disassembly (D'Souza et al, 2020). How this relates to the ability of ORP3 to downregulate PI(4)P or possibly to exchange PI(4)P with PC (D'Souza et al, 2020), remains unclear.…”
Section: Subfamily Iii: Orp3 Orp6 and Orp7mentioning
Lipids are precisely distributed in the eukaryotic cell where they help to define organelle identity and function, in addition to their structural role. Once synthesized, many lipids must be delivered to other compartments by non-vesicular routes, a process that is undertaken by proteins called Lipid Transfer Proteins (LTPs). OSBP and the closely-related ORP and Osh proteins constitute a major, evolutionarily conserved family of LTPs in eukaryotes. Most of these target one or more subcellular regions, and membrane contact sites in particular, where two organelle membranes are in close proximity. It was initially thought that such proteins were strictly dedicated to sterol sensing or transport. However, over the last decade, numerous studies have revealed that these proteins have many more functions, and we have expanded our understanding of their mechanisms. In particular, many of them are lipid exchangers that exploit PI(4)P or possibly other phosphoinositide gradients to directionally transfer sterol or PS between two compartments. Importantly, these transfer activities are tightly coupled to processes such as lipid metabolism, cellular signalling and vesicular trafficking. This review describes the molecular architecture of OSBP/ORP/Osh proteins, showing how their specific structural features and internal configurations impart unique cellular functions.
“…Complementing this work, a recent study links ORP3 activation by PKC and Ca 2þ entry with a mechanism implicated in focal adhesions dynamics. Once localized at ER-PM contact sites, ORP3 interacts with IQSec1,a guanine nucleotide exchange factors of Arf5 to trigger focal adhesion disassembly (D'Souza et al, 2020). How this relates to the ability of ORP3 to downregulate PI(4)P or possibly to exchange PI(4)P with PC (D'Souza et al, 2020), remains unclear.…”
Section: Subfamily Iii: Orp3 Orp6 and Orp7mentioning
Lipids are precisely distributed in the eukaryotic cell where they help to define organelle identity and function, in addition to their structural role. Once synthesized, many lipids must be delivered to other compartments by non-vesicular routes, a process that is undertaken by proteins called Lipid Transfer Proteins (LTPs). OSBP and the closely-related ORP and Osh proteins constitute a major, evolutionarily conserved family of LTPs in eukaryotes. Most of these target one or more subcellular regions, and membrane contact sites in particular, where two organelle membranes are in close proximity. It was initially thought that such proteins were strictly dedicated to sterol sensing or transport. However, over the last decade, numerous studies have revealed that these proteins have many more functions, and we have expanded our understanding of their mechanisms. In particular, many of them are lipid exchangers that exploit PI(4)P or possibly other phosphoinositide gradients to directionally transfer sterol or PS between two compartments. Importantly, these transfer activities are tightly coupled to processes such as lipid metabolism, cellular signalling and vesicular trafficking. This review describes the molecular architecture of OSBP/ORP/Osh proteins, showing how their specific structural features and internal configurations impart unique cellular functions.
Cell shape is regulated by adhesion and by cytoskeletal and membrane proteins. Cell shape, adhesion and motility have a complex relationship and understanding them is important in understanding developmental patterning and diseases such as cancer. Here we show that the lipid kinase phosphatidylinositol 4-kinase III beta (PI4KIIIβ) regulates cell shape, migration and Focal Adhesion number. PI4KIIIβ generates phosphatidylinositol 4-phosphate from phosphatidylinositol and is highly expressed in a subset of human breast cancers. PI4KIIIβ and the PI4P it generates regulate a range of cellular functions, among them Golgi structure, fly fertility and Akt signaling. Here we show that loss of PI4KIIIβ expression decreases cell migration and alters cell shape in NIH3T3 fibroblasts. The changes are accompanied by an increase in the number of Focal Adhesions in cells lacking PI4KIIIβ. Furthermore, we find that PI4P-containing vesicles move to the migratory leading-edge during migration and that some of these vesicles tether to and fuse with FA. This suggests a novel regulatory role for PI4KIIIβ in cell adhesion and cell shape maintenance.
The endoplasmic reticulum (ER) forms direct membrane contact sites with the plasma membrane (PM) in eukaryotic cells. These ER-PM contact sites play essential roles in lipid homeostasis, ion dynamics, and cell signaling, which are carried out by protein-protein or protein-lipid interactions. Distinct tethering factors dynamically control the architecture of ER-PM junctions in response to intracellular signals or external stimuli. The physiological roles of ER-PM contact sites are dependent on a variety of regulators that individually or cooperatively perform functions in diverse cellular processes. This review focuses on proteins functioning at ER-PM contact sites and highlights the recent progress in their mechanisms and physiological roles.
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