The NBD domains of the MRP6/ABCC6

Multidrug resistance-associated protein 6 (MRP6/ABCC6) is a protein belonging to the ABC energy-dependent efflux pumps family which members share many characteristic structural features, including two membrane-spanning domains and two nucleotide-binding domains (NBD1 and NBD2) that function cooperatively but not equally bind and hydrolyze ATP [1]. In this family MRP6 plays an important physiological role as demonstrated by the fact that mutations in this gene cause Pseudoxanthoma elasticum (PXE) in humans, a recessive genetic disorder affecting connective tissues characterized by progressive mineralization of elastic fibers [2]. In the present study, NBD1 and NBD2 of MRP6 has been expressed in Escherichia coli, purified and structurally and functionally characterized. The CD spectra demonstrate the presence of -helices, -strands and turns. Both polypeptides are shown to be biologically active. NBD2 binds ATP with an affinity similar to NBD1 but the ATPase activity is significantly different in the isolated NBDs. The mixture of NBD2 and NBD1 exhibited an activity similar to the NBD2 alone, indicating that NBD1 and NBD2 form a heterodimer with the latter limiting ATP hydrolysis. These findings suggest that NBD1 has a higher tendency to form an active homodimer, which is also supported by in silico analysis of energy-minimized dimers of the homology models of the two domains. 1. Slot AJ, Molinski SV, Cole SP. Mammalian multidrug-resistance proteins (MRPs). Essays Biochem. 2011;50(1):179-207. 2. Kavukcuoglu NB, Li Q, Pleshko N, Uitto J. Connective tissue mineralization in Abcc6-/- mice, a model for pseudoxanthoma elasticum. Matrix Biol. 2012;31(4):246-52.