Identification and functional characterization of AP-2 complex subunit mu-A as a new member of antimicrobial protein.

AP-2 complex subunit mu-A (AP2M1A) is a component of the adaptor complexes that link clathrin to receptors in coated vesicles. It has recently been shown to be involved in the resistance to oxidative damage, challenging the conventional role of AP2M1A. Here we demonstrated that AP2M1A was a heparin-binding protein abundantly stored in eggs and embryos of zebrafish, and its gene expression was markedly up-regulated by LPS and LTA treatment. We also showed that recombinant AP2M1A (rAP2M1A) was not only able to interact with Gram-negative and Gram-positive bacteria as well as their signature molecules LPS and LTA, but also able to inhibit the growth of the bacteria. Additionally, we found that AP2M1A that contained 2 closely positioned heparin-binding motifs could also bind to LPS and LTA, and inhibit the bacterial growth. Both rAP2M1A and AP2M1A were shown to execute antibacterial activity by a combined action of destabilization/destruction of bacterial cell wall through interaction with LPS and LTA, disturbance of the usually polarized membrane through depolarization, and apoptosis/necrosis through intracellular ROS production. Finally, we showed that AP2M1A could protect zebrafish developing embryos/larvae against attack by the potential pathogen Aeromonas hydrophila. All these demonstrate for the first time that AP2M1A is a maternal antimicrobial protein previously uncharacterized. It also establishes a correlation between antibacterial activity and heparin-binding motifs.