Contribution of AP2 and AP180 to clathrin function in Dictyostelium discoideum
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AP2 complex protein is an essential clathrin adaptor protein during clathrin mediated endocytosis. However, this view has been challenged in simple organisms. To gain insight into this conflict, the role of AP2 in clathrin localization and other clathrin related processes were assessed in Dictyostelium discoideum. In Dictyostelium, deleting function AP2 caused mild phenotypes in clathrin membrane localization, cytokinesis, osmoregulation and cell development. This supported the idea that AP2 have significant roles in multicellular organisms but not in unicellular system. Clathrin mediated processes carries important function not only on the plasma membrane but also on some internal organelles. But clathrin coated vesicles on internal organelles are not as well studied as on the plasma membrane. To understand more of the clathrin coated vesicles on internal organelles, the clathrin coated vesicles on Dictyostelium discoideum contractile vacuole were studied. Contractile vacuole associated clathrin coated vesicles contained clathrin adaptor proteins AP2, AP180, and epsin but not Hip1r. The absence of AP180 or AP2 produced abnormal large vacuoles, but the absence of epsin did not cause any detectable contractile vacuole abnormality. The enlarged contractile vacuoles in AP180 minus cells were caused by excessive homotypic fusion among contractile vacuoles. Using both GST-pull down and immunostaining AP180 was identified as the possible adaptor protein for a contractile vacuole-associated SNARE protein, Vamp7B. Therefore recycling Vamp7B from contractile vacuole by AP180 through clathrin coated vesicles could be an efficient way to prevent excessive homotypic fusions among contractile vacuoles. Dictyostelium contractile vacuoles offer a valuable system to study clathrin coated vesicles on cell internal organelles.