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Fig. 4. Conveying lipids across the cytoplasm. (A) Mode of action of a START-only protein. In its unliganded form, the START domain interacts with the membrane through its C-terminal {alpha}4 helix. This interaction induces a conformational change and opens the lipid-binding pocket. When the lipid (yellow) is occupying the pocket, the protein conformation changes again and the lid is closed. The liganded form of the START protein must be targeted to an acceptor membrane to deliver its lipid. (B) Mode of action of MLN64. The MENTAL domain of MLN64 (blue) anchors it to endosome membranes, leaving its C-terminal START domain (green) in the cytoplasm. The START domain of MLN64 might work by extracting cholesterol (yellow) bound to its MENTAL domain from the late endosome membrane and transfer it to a closely positioned acceptor membrane. This model would allow significant and rapid cholesterol transfer.