Probing PtdIns4P’s localization

Hammond et al. develop a new probe to detect the phospholipid PtdIns4P in living cells.

Different cellular membranes have distinct protein and lipid compositions. The phospholipid phosphatidylinositol 4-phosphate (PtdIns4P) is best known for its role at the Golgi, where it helps recruit proteins involved in vesicular transport. PtdIns4P is also found at the plasma membrane, and some evidence suggests it could be synthesized on late endosomes and lysosomes as well. But the fluorescent probes used to detect PtdIns4P in living cells use the PtdIns4P-binding domains of Golgi proteins that also bind to other components of Golgi membranes. Accordingly, these probes preferentially label the Golgi-localized pool of PtdIns4P in vivo.

Hammond et al. constructed a new PtdIns4P biosensor using the PtdIns4P-binding (P4M) domain of a bacterial protein called SidM. GFP-P4M localized to the Golgi, plasma membrane, and late endosomes/lysosomes of living mammalian cells. This wide-ranging distribution was dependent on PtdIns4P because P4M labeling was lost when the PtdIns4P-hydrolyzing phosphatase Sac1 was targeted to any of these organelles. On the other hand, mistargeting the PtdIns4-synthesizing enzyme PI4KA was sufficient to recruit P4M to additional cell membranes.

PtdIns4P’s widespread distribution means that it can’t be a simple marker of organelle identity. Instead, lead author Gerry Hammond speculates that the phospholipid may determine the distribution of other membrane components by recruiting proteins involved in vesicular transport and through a potential function in nonvesicular lipid transport between organelles. The P4M probe should help researchers investigate both this and the many other functions of PtdIns4P.