The mechanism of Ca²⁺-dependent recognition of Alix by ALG-2: insights from X-ray crystal structures

Alix [ALG-2 (apoptosis-linked gene 2)-interacting protein X] was originally identified as a protein that interacts with ALG-2, a member of the penta-EF-hand Ca²⁺-binding protein family. ALG-2 binds to its C-terminal proline-rich region that contains four tandem repeats of PXY (where X represents an uncharged amino acid). Recent X-ray crystal structural analyses of the Ca²⁺-free and Ca²⁺-bound forms of ALG-2, as well as the complex with an Alix oligopeptide, have revealed a mechanism of Ca²⁺-dependent binding of ALG-2 to its target protein. Binding of Ca²⁺ to EF3 (third EF-hand) enables the side chain of Arg¹²⁵, present in the loop connecting EF3 and EF4 (fourth EF-hand), to move sufficiently to make a primary hydrophobic pocket accessible to the critical PPYP (Pro-Pro-Tyr-Pro) motif in Alix, which partially overlaps with the GPP (Gly-Pro-Pro) motif for binding to Cep55 (centrosome protein of 55 kDa). The fact that ALG-2 forms a homodimer and each monomer has one peptide-binding site indicates the possibility that ALG-2 bridges two interacting proteins, including Alix and Tsg101 (tumour susceptibility gene 101), and functions as a Ca²⁺-dependent adaptor protein.