Selective arylation of cysteine-237 of rabbit muscle aldolase with 4-chloro-7-nitrobenzofurazan

Fluorescence studies on plasminogen activator inhibitor 1: reactive centre cysteine mutants remain active after fluorophore attachment

To investigate structural-functional aspects of plasminogen activator inhibitor 1 (PAI-1) we have taken advantage of the lack of cysteines in the PAI-1 molecule and replaced Ser344 (P3) and Asn329 (P18) with cysteine residues, thereby creating unique attachment sites for extrinsic fluorescent probes. After expression in E. coli and purification to homogeneity, both of the mutant proteins were found to have similar biochemical characteristics as wild type PAI-1 (wtPAI-1). Following labelling with 4-chloro-7-nitrobenzofurazan (NBD) and 2-(4'-iodoacetamido-anilino)naphtalene-6-sulfonic acid (IAANS) the mutant inhibitors showed similar inhibitory activities and heat stability as wtPAI-1. The purified complex between uPA and NBD-labelled P3cys mutant was found to be extremely stable, suggesting that no slow cleavage or reversible reaction occurs in complexes that have been properly formed. The rate of labelling of both mutants was decreased when the mutants were in the latent form indicating that these cysteine residues may be less accessible in the latent configuration. The PAI-1 mutants labelled with both NBD and IAANS could convert from the active to the latent form, but P3cys labelled with the larger IAANS chromophore showed a two fold decrease in the rate of conversion to latency, suggesting that a large chromophore in the P3 position may interfere with the active to latent conversion. The fluorescence spectra of the two NBD labelled mutants were similar, but the intensity was three times higher for the P3cys mutant than for P18cys. No significant spectral changes could be seen when the P3cys mutant was transferred to latency. In contrast, the P18cys mutant showed a major change in the excitation spectra characteristic of migration of the NBD chromophore from a thiol to an amine. Complex formation with uPA had no effect on the fluorescence spectrum of P18cys-NBD while the spectrum of P3cys-NBD revealed changes consistent with a restriction of the mobility of NBD probe in the uPA-PAI-1 complex.

Fluorescence resonance energy transfer studies on the proximity between lysine-107 and cysteine-239 in rabbit muscle aldolase

Spatial relationships between Lys-107, which binds the C-6 phosphate group of the substrate, and fast-reacting Cys-239, located outside the active site of rabbit muscle aldolase, were studied by means of resonance energy transfer. The Lys-107 residue was covalently linked to pyridoxal phosphate (fluorescence donor) and the Cys-239 residue was modified by 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (fluorescence acceptor). The energy transfer between donor and acceptor has been demonstrated. The steady-state and the lifetime measurements indicate that in solution the distance between Lys-107 and Cys-239 in the aldolase molecule is 12.4 A assuming chi 2 = 2/3.

Aldolase exists in both the fluid and solid phases of cytoplasm

We have prepared a functional fluorescent analogue of the glycolytic enzyme aldolase (rhodamine [Rh]-aldolase), using the succinimidyl ester of carboxytetramethyl-rhodamine. Fluorescence redistribution after photobleaching measurements of the diffusion coefficient of Rh-aldolase in aqueous solutions gave a value of 4.7 x 10(-7) cm2/S, and no immobile fraction. In the presence of filamentous actin, there was a 4.5-fold reduction in diffusion coefficient, as well as a 36% immobile fraction, demonstrating binding of Rh-aldolase to actin. However, in the presence of a 100-fold molar excess of its substrate, fructose 1,6-diphosphate, both the mobile fraction and diffusion coefficient of Rh-aldolase returned to control levels, indicating competition between substrate binding and actin cross-linking. When Rh-aldolase was microinjected into Swiss 3T3 cells, a relatively uniform intracellular distribution of fluorescence was observed. However, there were significant spatial differences in the in vivo diffusion coefficient and mobile fraction of Rh-aldolase measured with fluorescence redistribution after photobleaching. In the perinuclear region, we measured an apparent cytoplasmic diffusion coefficient of 1.1 x 10(-7) cm2/s with a 23% immobile fraction; while measurements in the cell periphery gave a value of 5.7 x 10(-8) cm2/s, with no immobile fraction. Ratio imaging of Rh-aldolase and FITC-dextran indicated that FITC-dextran was relatively excluded excluded from stress fiber domains. We interpret these data as evidence for the partitioning of aldolase between a soluble fraction in the fluid phase and a fraction associated with the solid phase of cytoplasm. The partitioning of aldolase and other glycolytic enzymes between the fluid and solid phases of cytoplasm could play a fundamental role in the control of glycolysis, the organization of cytoplasm, and cell motility. The concepts and experimental approaches described in this study can be applied to other cellular biochemical processes.