Cloning, Escherichia coli expression, and phase-transition chromatography-based purification of recombinant rabbit heart mitochondrial creatine kinase

Evidence of proteolipid domain formation in an inner mitochondrial membrane mimicking model

BACKGROUND

Mitochondrial creatine kinase (mtCK) is highly abundant in mitochondria; its quantity is equimolecular to the Adenylic Nucleotide Translocator and represents 1% of the mitochondrial proteins. It is a multitask protein localized in the mitochondria intermembrane space where it binds to the specific cardiolipin (CL) phospholipid. If mtCK was initially thought to be exclusively implicated in energy transfer between mitochondria and cytosol through a mechanism referred to as the phosphocreatine shuttle, several recent studies suggested an additional role in maintaining mitochondria membrane structure.

METHODS

To further characterized mtCK binding process we used multiphoton excitation fluorescence microscopy coupled with Giant Unilamellar Vesicles (GUV) and laurdan as fluorescence probe.

RESULTS

We gathered structural and dynamical information on the molecular events occurring during the binding of mtCK to the mitochondria inner membrane. We present the first visualization of mtCK-induced CL segregation on a bilayer model forming micrometer-size proteolipid domains at the surface of the GUV. Those microdomains, which only occurred when CL is included in the lipid mixture, were accompanied by the formation of protein multimolecular assembly, vesicle clamping, and changes in both vesicle curvature and membrane fluidity CONCLUSION: Those results highlighted the importance of the highly abundant mtCK in the lateral organization of the mitochondrial inner membrane.

GENERAL SIGNIFICANCE

Microdomains were induced in mitochondria-mimicking membranes composed of natural phospholipids without cholesterol and/or sphingolipids differing from the proposed cytoplasmic membrane rafts. Those findings as well as membrane curvature modification were discussed in relation with protein-membrane interaction and protein cluster involvement in membrane morphology.

Protein "amyloid-like" networks at the phospholipid membrane formed by 4-hydroxy-2-nonenal-modified mitochondrial creatine kinase

4-Hydroxy-2-nonenal (4-HNE) is a reactive aldehyde and a lipid peroxidation product formed in biological tissues under physiological and pathological conditions. Its concentration increases with oxidative stress and induces deleterious modifications of proteins and membranes. Mitochondrial and cytosolic isoforms of creatine kinase were previously shown to be affected by 4-HNE. In the present study, we analyzed the effect of 4-HNE on mitochondrial creatine kinase, an abundant protein from the mitochondrial intermembrane space with a key role in mitochondrial physiology. We show that this effect is double: 4-HNE induces a step-wise loss of creatine kinase activity together with a fast protein aggregation. Protein-membrane interaction is affected and amyloid-like networks formed on the biomimetic membrane. These fibrils may disturb mitochondrial organisation both at the membrane and in the inter membrane space.

Mitochondrial creatine kinase interaction with cardiolipin-containing biomimetic membranes is a two-step process involving adsorption and insertion

Mitochondrial creatine kinase (mtCK) binding to the mitochondrial inner membrane largely determines its biological functions in cellular energy homeostasis, mitochondrial physiology, and dynamics. The membrane binding mechanism is, however, not completely understood. Recent data suggest that a hydrophobic component is involved in mtCK binding to cardiolipin at the outer face of the inner mitochondrial membrane, in addition to the well known electrostatically driven process. In this manuscript, using an electrochemical method derived from alternating current polarography for differential capacity measurements, we distinctly reveal that protein-cardiolipin interaction has a two-step mechanism. For short incubation time, protein adsorption to the phospholipid charged headgroup was the only process detected, whereas on a longer time scale evidence of protein insertion was observed.

Mitochondrial creatine kinase binding to phospholipid monolayers induces cardiolipin segregation

It is well established that the octameric mitochondrial form of creatine kinase (mtCK) binds to the outer face of the inner mitochondrial membrane mainly via electrostatic interactions with cardiolipin (CL). However, little is known about the consequences of these interactions on membrane and protein levels. Brewster angle microscopy investigations provide, for the first time to our knowledge, images indicating that mtCK binding induced cluster formation on CL monolayers. The thickness of the clusters (10-12 nm) corresponds to the theoretical height of the mtCK-CL complex. Protein insertion into a condensed CL film, together with monolayer stabilization after protein addition, was observed by means of differential capacity measurements. Polarization modulation infrared reflection-absorption spectroscopy showed that the mean orientation of alpha-helices within the protein shifted upon CL binding from 30 degrees to 45 degrees with respect to the interface plane, demonstrating protein domain movements. A comparison of data obtained with CL and phosphatidylcholine/phosphatidylethanolamine/CL (2:1:1) monolayers indicates that mtCK is able to selectively recruit CL molecules within the mixed monolayer, consolidating and changing the morphology of the interfacial film. Therefore, CL-rich domains induced by mtCK binding could modulate mitochondrial inner membrane morphology into a raft-like organization and influence essential steps of mitochondria-mediated apoptosis.

Morphology modifications in negatively charged lipid monolayers upon mitochondrial creatine kinase binding

Mitochondrial creatine kinase (mtCK) may participate to membrane organization at the mitochondrial level by modulating lipid state and fluidity. The effect of the protein on lipid phase behaviour of different acyl chain length phosphatidylglycerol monolayers was analyzed from pressure-area isotherms and from the compressional modulus variation with respect to the surface pressure. Monolayer morphology was visualized by Brewster angle microscopy. No condensation effect was visible on dimyristoylphosphatidylglycerol (DMPG). For the other PG monolayers tested, dipalmitoylphosphatidylglycerol (DPPG) and distearoylphosphatidylglycerol (DSPG), mtCK facilitated the formation of a liquid condensed phase. The effect depended on the surface pressure at which transition phase occurred. The effect of mtCK was more pronounced for tetramyristoylcardiolipin (TMCL) monolayers, as liquid condensed regions appeared 10 mN/m below the transition phase of the pure TMCL monolayer. The observed domains were circular and rather uniform, indicating a stabilization of the condensed phase. The same effect, namely an overall condensation of the monolayer with formation of circular domains, was observed upon protein injection beneath TMCL monolayers in different condensation states at constant area. MtCK ability to induce and stabilize a LC phase on monolayers could have important consequences in membrane organization and emphasize its structural role at mitochondrial level.

Mitochondrial creatine kinase adsorption to biomimetic membranes: a Langmuir monolayer study

Interaction of mitochondrial creatine kinase (mtCK) with either synthetic or natural zwitterionic or acidic phospholipids was monitored by surface pressure measurements. Injection of mtCK beneath a monolayer at very low surface pressure results in a large increase in the apparent area per lipid molecule reflecting the intrinsic surface activity of the protein. This effect is particularly pronounced with anionic phospholipid-containing films. Upon compression to high lateral pressure, the protein is squeezed out of the lipid monolayer. On the contrary, mtCK injected beneath a monolayer compressed at 30 mN/m, does not insert into the monolayer but is concentrated below the surface by anionic phospholipids as evidenced by the immediate and strong increase in the apparent molecular area occurring upon decompression. Below 8 mN/m the protein adsorbs to the interface and remains intercalated until the lateral pressure increases again. The critical pressure of insertion is higher for anionic lipid-containing monolayers than for films containing only zwitterionic phospholipids. In the former case it is markedly diminished by NaCl. The adsorption of mtCK depends on the percentage of negative charges carried by the monolayer and is reduced by increasing NaCl concentrations. However, the residual interaction existing in the absence of a global negative charge on the membrane may indicate that this interaction also involves a hydrophobic component.

Interfacial behavior of cytoplasmic and mitochondrial creatine kinase oligomeric states

Adsorption to the air/water interface of isoenzymes of creatine kinase was investigated using surface pressure-area isotherms and Brewster angle microscopy (BAM) observations. Octameric mitochondrial creatine kinase (mtCK) exhibits a significant affinity for the air/water interface. Whatever the mode of formation of the interfacial film, i.e., injection of the protein in the subphase or spreading onto the buffer surface, the final arrangement and conformation adopted by mtCK molecules lead to a similar result. In contrast, the dimeric isoenzymes mtCK and cytosolic MMCK do not induce any surface pressure variation. However, when the subphase contains 0.3M NaCl, both isoenzymes adsorb to the interface. When treated with 0.8 or 3M GdnHCl, muscle creatine kinase (MMCK) becomes surface active and occupies a greater surface than mtCK. This result contrasts with previous observations, often derived from monomeric proteins, that their surface activity is increased upon unfolding. It underlines the possible influence exerted by the protein oligomeric state on its interfacial activity. At a subphase pH of 8.8, which corresponds to the pI of octameric mtCK, the profiles of the isotherms obtained with dimeric and octameric states and the resistance to compression of the protein monolayers are significantly affected when compared to those recorded at pH 7.4. These data suggest that the octamer is more hydrophobic than the dimer and may contribute to explaining why octamers bind to the inner mitochondrial membrane while dimers do not.

Expression of Torpedo californica creatine kinase in Escherichia coli and purification from inclusion bodies

The pET17 expression vector was used to express creatine kinase from the electric organ of Torpedo californica as inclusion bodies in Escherichia coli BL21(DE3) cells. The insoluble aggregate was dissolved in 8M urea and, following extraction with Triton X-100, the enzyme was refolded by dialysis against Tris buffer (pH 8.0) containing 0.2M NaCl. After two buffer changes, chromatography on Blue Sepharose was used as a final step in the purification procedure. Approximately 54mg active protein was recovered from a 1L culture and the refolded enzyme had a specific activity of 75U/mg. The molecular mass of the purified protein was consistent with that predicted from the amino acid sequence and the CD spectrum of the refolded enzyme was essentially identical to that of creatine kinase from human muscle (HMCK). The K(m) values of ATP and ADP were also similar to those of HMCK, while the K(m) values for both phosphocreatine and creatine were approximately 5-10-fold higher. The purification described here is in marked contrast with earlier attempts at purification of this isozyme where, in a process yielding less than 1mg/L culture, enzyme with a specific activity of ca. 5U/mg was obtained.

Mitochondrial creatine kinase binding to liposomes and vesicle aggregation: effect of cleavage by proteinase K

Mitochondrial creatine kinase and its proteinase K nicked-derivative interaction with liposomes induced slight secondary structure changes evidenced by infrared spectra. In nondenaturing conditions, the N-terminal (K1) and the C-terminal (K2) fragments remained associated with each other and bound to liposomes. When the two fragments were separated by denaturation, K2 was soluble, whereas most of K1 was adsorbed onto liposomes. The three-dimensional structure of uncleaved mtCK suggests that the C-terminal moiety, which contains positively charged surface residues, interacted with membranes. After denaturation and renaturation of the nicked enzyme, both peptides did not refold properly and did not reassociate with each other. The misfolded K1 fragment bound to the membrane through a stretch of positive residues, which were buried in the native enzyme. The lack of binding of the ill-folded K2 peptide could be related to the disruption of the optimal disposition of its positive charges, responsible for the correct interaction of native mtCK with membrane.