Expression and purification of dalcochinase, a β-glucosidase from Dalbergia cochinchinensis Pierre, in yeast and bacterial hosts

The coding sequence of the mature dalcochinase, a beta-glucosidase from Dalbergia cochinchinensis Pierre, was cloned and expressed in various systems. Expression in Escherichia coli resulted in an insoluble protein, which could be made soluble by co-expression with bacterial chaperonin GroESL. However, the enzyme had no activity. Recombinant expression in Pichia pastoris and Saccharomyces cerevisiae yielded an active enzyme. Dalcochinase was expressed under methanol induction in P. pastoris, since this was much more efficient than constitutive expression in P. pastoris or in S. cerevisiae. Addition of 0.5% casamino acids to the culture medium stabilized the pH of the culture and increased the protein yield by 3- to 5-folds. Insertion of a polyhistidine-tag either after the N-terminal alpha factor signal sequence or at the C-terminus failed to assist in purification by immobilized metal-ion affinity chromatography (IMAC) due to post-translational processing at both termini. A new construct of dalcochinase with an N-terminal truncation following the propeptide and eight histidine residues enabled its purification by IMAC, following hydrophobic interaction chromatography. The purified recombinant dalcochinase was apparently composed of differently post-translationally modified forms, but had kinetic properties and pH and temperature optima comparable to natural dalcochinase. The procedures reported here overcome the limitation in enzyme supply from natural sources, and allow further studies on structure-function relationships in this enzyme.

The hydrophobic domain 26 of human tropoelastin is unstructured in solution

Elastin is the protein responsible for the elastic properties of vertebrate tissue. Very little is currently known about the structure of elastin or of its soluble precursor tropoelastin. We have used high-resolution solution NMR methods to probe the conformational preferences of a conserved hydrophobic region in tropoelastin, domain 26 (D26). Using a combination of homonuclear, 15N-separated and triple resonance experiments, we have obtained essentially full chemical shift assignments for D26 at 278K. An analysis of secondary chemical shift changes, as well as NOE and 15N relaxation data, leads us to conclude that this domain is essentially unstructured in solution and does not interact with intact tropoelastin. D26 does not display exposed hydrophobic clusters, as expected for a fully unfolded protein and commensurate with an absence of flexible structural motifs, as identified by lack of binding of the fluorescent probe 4,4'-dianilino-1,1'-binaphthyl-5,5'-disulfonic acid. Sedimentation equilibrium data establish that this domain is strictly monomeric in solution. NMR spectra recorded at 278 and 308K indicate that no significant structural changes occur for this domain over the temperature range 278-308K, in contrast to the characteristic coacervation behavior that is observed for the full-length protein.

Hydrophobic domains of human tropoelastin interact in a context-dependent manner

Tropoelastin is the soluble precursor of elastin, the major component of the extracellular elastic fiber. Tropoelastin undergoes self-association via an inverse temperature transition termed coacervation, which is a crucial step in elastogenesis. Coacervation of tropoelastin takes place through multiple intermolecular interactions of its hydrophobic domains. Previous work has implicated those hydrophobic domains located near the center of the polypeptide as playing a dominant role in coacervation. Short constructs of domains 18, 20, 24, and a mutated form of domain 26 were largely disordered at 20 degrees C but displayed increased order on heating that was consistent with the formation of beta-structures. However, their conformational transitions were not sensitive to physiological temperature in contrast to the observed behavior of the native domain 26. A polypeptide consisting of domains 17-27 of tropoelastin coacervated at temperatures above 60 degrees C, whereas individually expressed hydrophobic regions were not capable of coacervation. We conclude that coacervation depends on the hydrophobicity of the molecule and, by inference, the number of hydrophobic domains. Tropoelastin mutants were constructed to contain a Pro --> Ala mutation in domain 26, separate deletions of domains 18 and 26, and a displacement of domain 26. These constructs displayed unequal capacities for coacervation, even when they contained the same number of hydrophobic regions and comparable levels of secondary structure. Thus, the capability for coacervation is determined by contributions from individual hydrophobic domains for which function should be considered in the context of their positions in the intact tropoelastin molecule.

Thermodynamic and Hydrodynamic Properties of Human Tropoelastin

Tropoelastin is the soluble precursor of elastin that bestows tissue elasticity in vertebrates. Tropoelastin is soluble at 20 degrees C in phosphate-buffered saline, pH 7.4, but at 37 degrees C equilibrium is established between soluble protein and insoluble coacervate. Sedimentation equilibrium studies performed before (20 degrees C) and after (37 degrees C) coacervation showed that the soluble component was strictly monomeric. Sedimentation velocity experiments revealed that at both temperatures soluble tropoelastin exists as two independently sedimenting monomeric species present in approximately equal concentrations. Species 1 had a frictional ratio at both temperatures of approximately 2.2, suggesting a very highly expanded or asymmetric protein. Species 2 displayed a frictional ratio at 20 degrees C of 1.4 that increased to 1.7 at 37 degrees C, indicating a compact and symmetrical conformation that expanded or became asymmetric at the higher temperature. The slow interconversion of the two monomeric species contrasts with the rapid and reversible process of coacervation suggesting both efficiently incorporate into the coacervate. A hydrated protein of equivalent molecular weight modeled as a sphere and a flexible chain was predicted to have a frictional ratio of 1.2 and 1.6, respectively. Tropoelastin appeared as a single species when studied by pulsed field-gradient spin-echo NMR, but computer modeling showed that the method was insensitive to the presence of two species of equal concentration having similar diffusion coefficients. Scintillation proximity assays using radiolabeled tropoelastin and sedimentation analysis showed that the coacervation at 37 degrees C was a highly cooperative monomer-n-mer self-association. A critical concentration of 3.4 g/liter was obtained when the coacervate was modeled as a helical polymer formed from the monomers via oligomeric intermediates.