MLN64 (metastatic lymph node 64) and MENTHO (MLN64 N-terminal homologue) are two late-endosomal proteins that share a conserved region of four transmembrane helices with three short intervening loops called the MENTAL domain (MLN64 N-terminal domain). This domain mediates MLN64 and MENTHO homo- and hetero-interactions, targets both proteins to late endosomes and binds cholesterol in vivo. In addition to the MENTAL domain, MLN64 contains a cholesterol-specific START domain [StAR (steroidogenic acute regulatory protein)-related lipid transfer domain]. The START domain is a protein module of approx. 210 residues that binds lipids, including sterols, and is present in 15 distinct proteins in mammals. Thus MLN64 and MENTHO define discrete cholesterol-containing subdomains within the membrane of late endosomes where they may function in cholesterol transport. The MENTAL domain might serve to maintain cholesterol at the membrane of late endosomes prior to its shuttle to cytoplasmic acceptor(s) through the START domain.

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