THE N-TERMINAL DOMAIN OF ANNEXIN 2 SERVES AS A SECONDARY BINDING SITE DURING MEMBRANE BRIDGING
Résumé
Annexin A2 (AnxA2) is a Ca 2+-and acidic phospholipid-binding protein involved in many cellular processes. It undergoes Ca 2+-mediated membrane bridging at neutral pH and has been demonstrated to be involved in an H +-mediated mechanism leading to a novel AnxA2-membrane complex structure. We used fluorescence techniques to characterize this H +-dependent mechanism at the molecular level, in particular the involvement of the AnxA2 N-terminal domain. This domain was labeled at Cys8 either with acrylodan or pyrene-maleimide fluorescent probes. Steady-state and time-resolved fluorescence analysis for acrylodan and fluorescence quenching by doxyl-labelled phospholipids revealed direct interaction between the N-terminal domain and the membrane. The absence of pyrene excimer suggested that interactions between N-termini are not involved in the H +-mediated mechanism. These findings differ from those previously observed for the Ca 2+-mediated mechanism. Protein titration experiments showed that the protein concentration for half-maximal membrane aggregation was twice for Ca 2+-mediated compared to H +-mediated aggregation, suggesting that AnxA2 was able to bridge membranes either as a dimer or as a monomer, respectively. A N-terminally deleted AnxA2 was two to three times less efficent than the wild type protein for H+-mediated membrane aggregation. We propose a model of AnxA2-membrane assemblies, highlighting the different roles of the N-terminal domain in the H +-and Ca 2+-mediated membrane bridging mechanisms.
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