Pitstop 2 is a potent inhibitor of clathrin-independent endocytosis

Clathrin independent endocytosis (CIE) is a form of endocytosis present in all cells that mediates the entry of nutrients, macromolecules and membrane proteins into cells. When compared to clathrin-dependent endocytosis (CDE), however, much less is known about the machinery involved in forming CIE endosomes. One way to distinguish CIE from CDE has been to deplete cells of coat proteins involved in CDE such as clathrin or the dynamin GTPase, leading to a block of CDE but not CIE. A drawback of such genetic manipulations is that depletion of proteins important for mediating CDE over a period of days can have complex indirect effects on cellular function. The identification of chemical compounds that specifically and rapidly block CDE or CIE would facilitate the determination of whether a process involved CDE or CIE. To date, all of those compounds have targeted CDE. Dynasore and the dynoles specifically target and block dynamin activity thus inhibiting CDE but not most forms of CIE. Recently, a new compound called pitstop 2 was identified as an inhibitor of the interaction of amphiphysin with the amino terminal domain of clathrin, and shown to inhibit CDE in cells. Here we show that pitstop 2 is also a potent inhibitor of CIE. The effects of pitstop 2 are not restricted to inhibition of clathrin since knockdown of clathrin fails to rescue the inhibition of endocytosis of CIE proteins by the drug. Thus pitstop 2 has additional cellular targets besides the amino terminal domain of clathrin and thus cannot be used to distinguish CIE from CDE.