MLN64 Transport to the Late Endosome Is Regulated by Binding to 14-3-3 via a Non-canonical Binding Site

MLN64 is an integral membrane protein localized to the late endosome and plasma membrane that is thought to function as a mediator of cholesterol transport from endosomal membranes to the plasma membrane and/or mitochondria. The protein consists of two distinct domains: an N-terminal membrane-spanning domain that shares homology with the MENTHO protein and a C-terminal steroidogenic acute regulatory protein (StAR)-related lipid transfer (START) domain that binds cholesterol. To further characterize the MLN64 protein, full-length and truncated proteins were overexpressed in cells and the effects on MLN64 trafficking and endosomal morphology were observed. To gain insight into MLN64 function, affinity chromatography and mass spectrometric techniques were used to identify potential MLN64 interacting partners. Of the 15 candidate proteins identified, 14-3-3 was chosen for further characterization. We show that MLN64 interacts with 14-3-3 in vitro as well as in vivo and that the strength of the interaction is dependent on the 14-3-3 isoform. Furthermore, blocking the interaction through the use of a 14-3-3 antagonist or MLN64 mutagenesis delays the trafficking of MLN64 to the late endosome and also results in the dispersal of endocytic vesicles to the cell periphery. Taken together, these studies have determined that MLN64 is a novel 14-3-3 binding protein and indicate that 14-3-3 plays a role in the endosomal trafficking of MLN64. Furthermore, these studies suggest that 14-3-3 may be the link by which MLN64 exerts its effects on the actin-mediated endosome dynamics.