Structure of a dimeric ABC transporter

Abstract

ATP binding cassette (ABC) proteins generally couple ATP hydrolysis to the active transport of solutes across cellular membranes. All ABC proteins contain a core structure of two transmembrane domains (TMD1, TMD2) and two nucleotide binding domains (NBD1, NBD2), and many ABC proteins contain additional domains. Some members of the C subfamily of ABC (ABCC) proteins, such as the multidrug resistant protein 1 (MRP1), contain an N-terminal transmembrane domain (TMD0) and L0 linker that regulate transport activity and cellular trafficking, and mediate interactions with other proteins. Regulation can also be imparted by phosphorylation, proteolytic processing, and/or oligomerization of the proteins. Here we present the structure of yeast cadmium factor 1 (Ycf1p), a homologue of MRP1, in its mature form following cleavage by the yeast protease Pep4p. Remarkably, proteolytically cleaved Ycf1p forms a well-ordered dimer, with some monomeric particles also present in solution. Numerous other ABC proteins have been proposed to form dimers but no high-resolution structures have been reported. The monomeric and dimeric Ycf1p species are differentially phosphorylated at the intrinsically disordered regulatory (R) region, which links NBD1 to TMD2, and possess different ATPase activities indicating that dimerization affects the function of the protein. Protein-protein interactions involving TMD0, the L0 linker, and the R region mediate contacts between Ycf1p protomers in the dimer. In addition, cryo-EM density is observed for lipids at the interface between protomers, which suggests that lipids stabilize the dimer. The Ycf1p dimer structure is consistent with proposed dimerization interfaces of other ABCC dimers, such as MRP1.