Cholesterol-binding domain CRAC motif

Although the third amino acid that defines the CRAC domain (Lys-79 or Lys-81 in this case) is not directly involved in cholesterol binding, it stabilizes the orientation of Tyr-77, resulting in a remarkable fit of cholesterol for this CRAC domain. This interaction illustrates how well the algorithm performs. Nevertheless, the whole motif is rather polar than apolar because the nondefined X residues of this CRAC domain are chiefly arginine and lysine. Consequently, this domain is not located inside the membrane, but in the cytoplasm. Thus, despite the fact that this domain can theoretically bind cholesterol with high affinity, as correctly predicted by the CRAC algorithm, its location outside the membrane precludes any functional interaction with membrane cholesterol. 

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