Next, we measured the effect of disease-associated mutations on the ATPase activity of ABCA4. WT and ABCA4 variants were solubilized in CHAPS, purified by immunoaffinity chromatography, and subsequently reconstituted into PE-containing liposomes at similar protein concentrations. The ATPase activity of the mutants in the presence and absence of N-Ret-PE substrate is shown in Figure 6A, ​A,6B.6B. As previously reported,25,47 addition of 40 μM all-trans retinal to WT ABCA4 resulted in a 1.8- to 2.5-fold increase in ATPase activity (Fig. 6). The ATPase activity of the mutants was measured at the same protein concentration as WT ABCA4 to determine the effect of the mutation on the functional activity of ABCA4. Five mutants (p.Val552Ile, p.Ala1038Val, p.Ala1357Thr, p.Ala1794Pro, and p.Leu2027Phe) showed reduced basal ATPase activity relative to WT ABCA4 (∼40%–85%), but this activity was stimulated 1.6- to 3.0-fold by the addition of all-trans retinal. On the other hand, p.Gly72Arg, p.Met448Lys, p.Leu541Pro, p.Gly1091Glu, p.Gly1961Glu, and p.Arg2077Trp variants showed drastically reduced basal activity with little or no substrate stimulation.Open in a separate windowFigure 6ATPase activity of ABCA4 variants. The ATPase activity of immunopurified and reconstituted ABCA4 variants was measured in the presence or absence of all-trans retinal. (A) Quantification of the basal and retinal-stimulated ATPase activity of ABCA4 variants normalized to WT basal ATPase activity. ATPase assays were carried out using similar concentrations of purified ABCA4. Data expressed as an average ± SD for n ≥ 3 independent experiments. (B) Representative curves of specific ATPase activity as a function of all-trans retinal concentration for WT and ABCA4 variants. (C) Relative basal ATPase activity of WT and A1794P using equal amounts of transfected HEK293T cells. Data expressed as an average ± SD. Measurements were done in triplicate.To more directly evaluate the expression and function of the p.Ala1794Pro variant, we transfected HEK293T cells separately with WT ABCA4 and the p.Ala1794Pro mutant cDNAs at similar levels. After solubilization in CHAPS buffer, the samples were subjected to high-speed centrifugation and the supernatant was reconstituted into liposomes for analysis of its basal and substrate activated ATPase activity. As shown in Figure 6C, the p.Ala1794Pro had a significantly reduced activity due largely to the low expression of this variant. These studies indicate that only a small fraction of the p.Ala1794Pro mutant folds into a functionally active protein and correlates well with the phenotype of patient 5.

Variants S206R and L541P have reduced basal ATPase and S206R shows little, and L541P shows no, stimulation by retinal (Fig. 4a).

The measurement of ATPase activity has been the only assay available to study the effects of mutations on ABCA4 function. We employed this assay to examine the effects of [L541P; A1038V], R602W and C1490Y mutations on in vitro ATP hydrolysis. Constructs containing wild-type and mutated ABCA4 cDNAs, tagged with the eight amino acid bovine opsin C-terminal epitope (1D4), were expressed in COS7 cells and proteins were purified on a 1D4 affinity column. CHAPS-solubilized ABCA4 was incubated subsequently with ATP, and the hydrolysis rate was estimated with the charcoal method (31).The rate of ATP hydrolysis of the complex allele [L541P; A1038V] was decreased to 68.1% of wild-type ABCA4 (Fig. 3).

G818EER51 ± 1363 ± 5111 ± 825 ± 312 ± 32Moderate/Severe

Analysis of the rates of hydrolysis of ATP and CTP in the mutant protein demonstrated that they were significantly reduced (Fig. 3). The results presented in Fig. 3A indicated that the ATPase function of G863A mutant protein was reduced ∼3-fold as compared with NBD1wt, indicating ∼70% of inhibition of the ATPase activity. TheVmax (ATPase) for G863A mutant was 128 pmol/min/mg and that of the wild-type NBD1 was 584 pmol/min/mg (Table II). A time-course analysis of ATP hydrolysis using 2.5 μg of protein (Fig. 3C) suggested the actual rates of ATP hydrolysis were attenuated 3-fold as a consequence of the mutation. We have earlier reported that the NBD1wt has significantly higher CTPase than ATPase activity (1717.Biswas, E.E.Biochemistry. 2001; 40:8181-8187CrossrefScopus (25)PubMedGoogle Scholar). However, in the mutant G863A protein the CTPase activity was reduced (Fig.3B). In this case, the CTP hydrolysis of G863A was reduced to ∼30% of the activity of NDB1wt. The Vmaxfor G863A mutant was 104 pmol/min/mg and that of the wild-type NBD1 was 376 pmol/min/mg (Table II).