Fig. 2: Molecular characterization and structural features of ABCA4.

a Representative ATPase activity as a function of ATP for purified ABCA4 in the presence of phosphatidylethanolamine (PE) alone (Basal activity) and in the presence of PE and all-trans retinal (ATR) to generate N-retinylidene-PE (N-Ret-PE). Data expressed as a mean ± SD for three replicate measurements. Three independent experiments gave similar results The ATPase-deficient variant (MM-ABCA4) in which a lysine residue in each Walker A motif is replaced with a methionine is shown as a control. Similar curves were generated in three independent experiments. b Topological model of ABCA4 showing the N-linked glycosylation sites (blue hexagons) and disulfide bridges in the exocytoplasmic domains (ECD). Nucleotide binding domains (NBD) together with the regulatory domains (RD) are on the cytoplasmic side of the membranes. Helices in the transmembrane domains (TMD) are presented as cylinders with TMD1 in dark green and TMD2 in light green. Each TMD contains two internal transverse helices (IH) and two external helices (EH) as part of a α-helix-turn-α-helix structure. c Overall structure of ABCA4 in the unbound state. The structure is represented as cartoon with the N- and C- terminal halves colored dark and light green, respectively. N-linked glycans are represented as pink sticks. d Schematic showing the dimensions of two stacked discs. ABCA4 only can be accommodated in the rim region due its elongated ECDs. Localization of rhodopsin in the flat part of the disc is shown for comparison.