Changes in the adenine nucleotide and inorganic phosphate content of Escherichia coli F1-ATPase during ATP synthesis in dimethyl sulphoxide

Functional interaction between the two halves of the photoreceptor-specific ATP binding cassette protein ABCR (ABCA4). Evidence for a non-exchangeable ADP in the first nucleotide binding domain

ABCR, also known as ABCA4, is a member of the superfamily of ATP binding cassette transporters that is believed to transport retinal or retinylidene-phosphatidylethanolamine across photoreceptor disk membranes. Mutations in the ABCR gene are responsible for Stargardt macular dystrophy and related retinal dystrophies that cause severe loss in vision. ABCR consists of two tandemly arranged halves each containing a membrane spanning segment followed by a large extracellular/lumen domain, a multi-spanning membrane domain, and a nucleotide binding domain (NBD). To define the role of each NBD, we examined the nucleotide binding and ATPase activities of the N and C halves of ABCR individually and co-expressed in COS-1 cells and derived from trypsin-cleaved ABCR in disk membranes. When disk membranes or membranes from co-transfected cells were photoaffinity labeled with 8-azido-ATP and 8-azido-ADP, only the NBD2 in the C-half bound and trapped the nucleotide. Co-expressed half-molecules displayed basal and retinal-stimulated ATPase activity similar to full-length ABCR. The individually expressed N-half displayed weak 8-azido-ATP labeling and low basal ATPase activity that was not stimulated by retinal, whereas the C-half did not bind ATP and exhibited little if any ATPase activity. Purified ABCR contained one tightly bound ADP, presumably in NBD1. Our results indicate that only NBD2 of ABCR binds and hydrolyzes ATP in the presence or absence of retinal. NBD1, containing a bound ADP, associates with NBD2 to play a crucial, non-catalytic role in ABCR function.

Properties of bound inorganic phosphate on bovine mitochondrial F1F0-ATP synthase

Beef-heart mitochondrial F1F0-ATP synthase contained six molecules of bound inorganic phosphate (Pi). This phosphate exchanged completely with exogenous 32Pi when the enzyme was exposed to 30% (v/v) dimethyl sulfoxide (DMSO) and then returned to a DMSO-free buffer (Beharry and Bragg 2001). Only two molecules were replaced by 32Pi when the enzyme was not pretreated with DMSO. These two molecules of 32Pi were not displaced from the enzyme by the treatment with 1 mM ATP. Similarly, two molecules of bound 32Pi remained on the DMSO-pretreated enzyme following addition of ATP, that is, four molecules of 32Pi were displaced by ATP. The ATP-resistant 32Pi was removed from the enzyme by pyrophosphate. It is proposed that these molecules of 32Pi are bound at an unfilled adenine nucleotide-binding noncatalytic site on the enzyme. Brief exposure of the enzyme loaded with two molecules of 32Pi to DMSO, followed by removal of the DMSO, resulted in the loss of the bound 32Pi and in the formation of two molecules of bound ATP from exogenous ADP. A third catalytic site on the enzyme was occupied by ATP, which could undergo a Pi <--> ATP exchange reaction with bound Pi. The presence of two catalytic sites containing bound Pi is consistent with the X-ray crystallographic structure of F1 (Bianchet, et al., 1998). Thus, five of the six molecules of bound Pi were accounted for. Three molecules of bound Pi were at catalytic sites and participated in ATP synthesis or Pi <--> ATP exchange. Two other molecules of bound Pi were present at a noncatalytic adenine nucleotide-binding site. The location and role of the remaining molecule of bound Pi remains to be established. We were unable to demonstrate, using chemical modification of sulfhydryl groups by iodoacetic acid, any gross difference in the conformation of F1F0 in DMSO-containing compared with DMSO-free buffers.

Phosphate exchange and ATP synthesis by DMSO-pretreated purified bovine mitochondrial ATP synthase

Purified soluble bovine mitochondrial F(1)F(o)-ATP synthase contained 2 mol of ATP, 2 mol of ADP and 6 mol of P(i)/mol. Incubation of this enzyme with 1 mM [(32)P]P(i) caused the exchange of 2 mol of P(i)/mol of F(1)F(o)-ATP synthase. The labelled phosphates were not displaced by ATP. Transfer of F(1)F(o)-ATP synthase to a buffer containing 30% (v/v) DMSO and 1 mM [(32)P]P(i) resulted in the loss of bound nucleotides with the retention of 1 mol of ATP/mol of F(1)F(o)-ATP synthase. Six molecules of [(32)P]P(i) were incorporated by exchange with the existing bound phosphate. Removal of the DMSO by passage of the enzyme through a centrifuged column of Sephadex G-50 resulted in the exchange of one molecule of bound [(32)P]P(i) into the bound ATP. Azide did not prevent this [(32)P]P(i)<-->ATP exchange reaction. The bound labelled ATP could be displaced from the enzyme by exogenous ATP. Addition of ADP to the DMSO-pretreated F(1)F(o)-ATP synthase in the original DMSO-free buffer resulted in the formation of an additional molecule of bound ATP. It was concluded that following pretreatment with and subsequent removal of DMSO the F(1)F(o)-ATP synthase contained one molecule of ATP at a catalytic site which was competent to carry out a phosphate-ATP exchange reaction using enzyme-bound inorganic radiolabelled phosphate. In the presence of ADP an additional molecule of labelled ATP was formed from enzyme-bound P(i) at a second catalytic site. The bound phosphate-ATP exchange reaction is not readily accommodated by current mechanisms for the ATP synthase.