Beschreibung:
Organelle identity may be assisted by a phosphoinositide (PI) code, wherein specific PI lipids act as molecular “signposts”, ensuring exclusive recruitment of binding proteins to the correct membrane compartment. One example is PtdIns4P, whose known effector proteins are recruited to the Golgi. However, the PI 4‐kinases (PI4K) that make PtdIns4P belie such a restricted function, since they act at the Golgi, plasma membrane (PM) and in lysosomal traffic. To reconcile this contradiction, we re‐examined the distribution of PtdIns4P with a novel biosensor derived from the P4M domain of L. pneumophila SidM. P4M localizes to the Golgi, PM and dynamic Rab7‐positive late endosomes/lysosomes. Chemically‐induced recruitment of PI phosphatases to these compartments demonstrates PtdIns4P is the lipid bound in each case. Whereas the bulk of another PI lipid, PtdIns(4,5)P2 is localized at the PM, it could be produced in the other PtdIns4P‐containing compartments by recruitment of a PtdIns4P 5‐kinase. P4M can also detect ectopically produced PtdIns4P on ER‐derived membranes. We show that each of the four PI4Ks overlap with pools of PtdIns4P, and the ER/Golgi localized PtdIns4P phosphatase, Sacm1l, hydrolyses both Golgi and PM pools. We conclude that PtdIns4P has a much more complex function than as a simple molecular signpost for the Golgi, and likely functions through several as yet unappreciated protein interactions.Grant Funding Source: Intramural Research Program of the Eunice Kennedy Shriver NICHD, NIH