The membrane minor phospholipid PI(4,5)P2 has long been implicated in endocytosis. Many endocytotic proteins contain binding sites for it, and, without this binding, endocytosis is impaired. Evidence for the enrichment of PI(4,5)P2 at sites of endocytosis, however, has been lacking. The team used a PI(4,5)P2- binding domain, tagged with a fluorescent protein, to follow the dynamics of PI(4,5)P2 during endocytosis in live cells.
They show that PI(4,5)P2 is present in patches on the membrane and that these patches move into the cell over time and disappear. To confirm this inward movement was indeed endocytosis, the team used a second fluorescently labeled endocytosis coat protein and showed that the two colors colocalized.
The disappearance of PI(4,5)P2 coincided with the recruitment of phosphoinositide phosphatase, which is known to break down PI(4,5)P2. In cells in which PI(4,5)P2 breakdown was impaired, the team saw abnormal membrane invaginations that are thought to be the sites of multiple attempts and failures at endocytosis.
Indeed, without PI(4,5)P2 turnover, the endocytotic machinery still arrived at the membrane patches but hung around for longer without completing their job. It thus appears that the breakdown of PI(4,5)P2 is required for the scission of endocytotic vesicles.