Annexin-V Binds to the Intracellular Part of the β5 Integrin Receptor Subunit

Bovine lactadherin binds to the αvβ3 and αvβ5 integrins in an RGD-dependent manner and also to anionic phospholipids. During the affinity purification of lactadherin binding receptors, a 35-kDa protein persistently coeluted with the αvβ5 integrin receptor. Subsequently, peptide mapping, amino acid sequencing, and mass spectrometry analysis identified this protein as bovine annexin-V. Annexin-V accompanied the integrin receptor eluted with either RGD peptide or with EDTA suggesting that annexin-V bound specifically to the αvβ5 integrin. To further investigate this putative interaction of annexin-V with the αvβ5 integrin receptor, human annexin-V and intracellular domains of the human αvβ5 integrin subunits were used in ligand blotting assays. Radiolabeled annexin-V showed weak binding to the intracellular part of β5 integrin subunit. However, by adding the aminophospholipid, phosphatidyl serine, the interaction with the β5 cytoplasmic peptide was enhanced many fold. Furthermore, the interaction was shown to be independent of phosphorylation, as annexin-V bound to unphosphorylated β5 peptide at a similar level to the phosphorylated peptide. Since binding of annexin-V to the αv integrin subunit tail was not detected, annexin-V was shown to associate specifically with the β5 cytoplasmic tail. Together these findings suggest a novel link between annexins and the integrin receptor family.