Phosphoinositide synthesis and distribution in eukaryotic cells . (A) The biochemical reactions contributing to the PI cycle. Agonists of receptors that couple to PLC activation lead to the hydrolysis of PI(4,5)P₂ in the PM (represented by the gray rectangle). Both products of the hydrolytic reaction that consumes PI(4,5)P₂, DG and InsP₃, are used as metabolic intermediates for the rapid resynthesis of PI. PI synthesis takes place within membranes of the ER (represented by the green rectangle) via a two-step enzymatic reaction that is catalyzed by the sequential activity of cytidine-diphosphate-DG (CDP-DG) synthase (CDS) and PI synthase (PIS; also called CDP-DG;myo-inositol 3-phospho-transferase [CDIPT]) using PA and myo-inositol as substrates, respectively. PI has to be transported from the ER to reach the PM, or other organelles, where it can be phosphorylated to generate PI4P by PI4K enzymes. PI4P can be further phosphorylated by PI4P-specific 5-kinases (type I PIP5Ks) to produce PI(4,5)P₂, primarily in the PM. At the same time, upon hydrolysis of resident PI(4,5)P₂, the DG formed in the PM is rapidly converted to PA by diacylglycerol kinases (DGKs) and it has been shown that the lipid transfer protein, Nir2, is involved in the transport of PA from the PM to the ER, while also transferring PI from the ER to the PM. It is possible, although not proven, that DG may also be directly transported from the PM and into the ER where is could also be converted to PA (not shown). (B) Cellular topology of the PI cycle reactions and lipid transport mechanisms. Enzymatic reactions are shown by blue arrows, whereas lipid transfer routes are shown with orange arrows. Note that the nonvesicular lipid counter-transport processes between the PM and ER and between the Golgi and the ER are driven by the PI4P gradient that is established by the resident PI4K and PI4P-phosphatase activities. Similar PI4P-driven lipid transport processes exist between the endosomes and the ER (not shown for simplicity). Oxysterol binding protein (OSBP) and its related proteins, the ORPs, have been found to carry out these lipid transport processes the efficiency of which requires the ER-localized Sac1 phosphatase that dephosphorylates PI4P within the ER to keep the bulk levels of this lipid low. Recent studies have highlighted the prominent PI and PI4P content of the Golgi, where PI4P is synthesized by four different PI4K enzymes, while there is a relatively low amount of PI in the PM, possibly as a result of rapid conversion to PI4P by PI4KA. The pathways by which bulk PI reaches the other organelles is yet to be identified, although there is evidence to suggest that the Nir2 protein could deliver PI in exchange for PA in ER–PM contacts. However, the mechanism by which PI4P present at the Golgi complex might reach the PM, as suggested by the present study by Myeong et al. (2020), also awaits identification. It is also possible that these transfer processes may involve both vesicular and nonvesicular lipid transfer components.