2-[(4-Azido-2-nitrophenyl)amino]ethyl triphosphate, a novel chromophoric and photoaffinity analog of ATP. Synthesis, characterization, and interaction with myosin subfragment 1

Abstract

A facile and high-yield analysis of a new ATP analogue, 2-[(4-azido-2-nitrophenyl)amino]ethyl triphosphate (NANTP), is described. NANTP and ATP are hydrolyzed by skeletal myosin subfragment 1 (SF1) at comparable rates in the presence of Ca2+, Mg2+, or NH4+-EDTA. NANTP is also cleaved but less readily by mitochondrial F1-ATPase and by (Na+ + K+)-ATPase from dog brain and hog kidney. F-Actin markedly activates NANTP cleavage by SF1 in the presence of Mg2+, suggesting that the diphosphate product NANDP is slow to be released from the enzyme. [.alpha.-32P]NANDP binds to a single site on SF1 (KA = 1 .times. 106 M-1) with an affinity identical with that of ADP. The absorption maximum of NANDP was shifted from 474 to 467 nm upon binding to SF1, suggesting that the purine binding site has a dielectric constant of about 45. NANDP was trapped in nearly stoichiometric amounts at the active site by cross-linking SH1 and SH2 with N,N''-p-phenylenedimaleimide (pPDM) or by chelation with cobalt(III) phenanthroline [Wells, J., and Yount, R. (1979) Proc, Natl. Acad. Sci. U.S.A. 76, 4966]. The trapped [.beta.-32P]NANDP.cntdot.SF1 complex, a like the comparable ADP.cntdot.SF1 complex, was stable for days at 0.degree. C and could be purified free of extraneous analogue by ammonium sulfate precipitation and gel filtration. Photolysis of the purified complex gave > 50% covalent incorporation of the trapped NANDP into the 95-kilodalton (kDa) heavy chain of SF1. Limited trypsinization and analysis by gel electrophoresis showed that > 95% of the bound label was associated with the 25-kDa NH2-terminal peptide. Without trapping, NANDP labeling of SF1 was nonspecific and was not prevented by addition of a large excess of ATP. This new approach of trapping photoaffinity analogues by cross-linking agents before photolysis may prove to be of general usefulness in increasing the specificity and extent of labeling of enzymes that undergo substrate-induced conformation changes.