The secretory pathway is involved in several vital cellular processes, including host-pathogen interactions, nutrient and gravity sensing, and protein sorting [1-3]. In the past years, a subfamily of P-type ATPases has been suggested to be involved in vesicle formation. P-type ATPases comprise a large family of membrane proteins involved in pumping different physiologically-relevant substrates across biological membranes [4]. The members of the P4 subfamily (also known as flippases) catalyze the energy-driven translocation of lipids necessary for establishing transbilayer lipid asymmetry [5], a feature necessary for correct functioning of the cells [6,7]. Deletion of one or more P4-ATPase genes causes defects in vesicle budding in various organisms [8-10] and some members of the yeast family have been shown to interact with the vesiculation machinery [11,12]. Thus, unraveling the key features of P4-ATPase functioning is crucial to understand the mechanisms underlying the whole secretory and endocytic pathways.
In the model plant Arabidopsis, 12 members of the P4-ATPase family have been described (ALA1-ALA12, for Aminophospholipid ATPase) [4]. In the past years, we have characterized several members of this family with respect to their localization, substrate specificity, physiological role and requirement for the presence of a ß-subunit [9,13-15]. At the moment we are working on understanding the mechanism of lipid trasnsport and the regulation of these pumps. In this context, we have recently completed the biochemical characterization of two ALA proteins: ALA2, a prevacuolar compartment-localized protein with an unusually tight specificity, and ALA10, a plasma membrane-localized protein with and unforeseen broad substrate specificity. Besides providing an insight into the mechanism of lipid translocation, our results suggest that the different transport features of these proteins might be related to their physiological function at the membrane where they are located.