Flippases: still more questions than answers.

Our understanding of flippase-mediated lipid translocation and membrane vesiculation, and the involvement of P-type ATPases in these processes is just beginning to emerge. The results obtained so far demonstrate significant complexity within this field and point to major tasks for future research. Most importantly, biochemical characterization of P(4)-ATPases is required in order to clarify whether these transporters indeed are capable of catalyzing transmembrane phospholipid flipping. The beta-subunit of P(4)-ATPases shows unexpected similarities between the beta- and gamma-subunits of the Na+/K+-ATPase. It is likely that these proteins provide a similar solution to similar problems, and might have adopted similar structures to accomplish these tasks. No P(4)-ATPases have been identified in the endoplasmic reticulum and it remains an intriguing possibility that, in this compartment, P(5A)-ATPases are functional homologues of P(4)-ATPases.