NMR studies of the mechanism of action and regulation of protein kinase

Integrated pH Measurement during Reaction Monitoring with Dual-Reception 1H-31P NMR Spectroscopy

Simultaneous detection of ¹H and ³¹P NMR signals through a dual-detection scheme with two receivers allows monitoring of both the signals of a molecule and the pH of the solution through the resonance of the inorganic phosphate. We evaluate here the method in terms of sensitivity and ease of implementation and show that the additional information obtained without any loss of information or increase in measuring time can be of practical importance in a number of biochemical systems.

Activation loop phosphorylation modulates Bruton's tyrosine kinase (Btk) kinase domain activity

Bruton's tyrosine kinase (Btk) plays a central role in signal transduction pathways regulating survival, activation, proliferation, and differentiation of B-lineage lymphoid cells. A number of cell signaling studies clearly show that Btk is activated by Lyn, a Src family kinase, through phosphorylation on activation loop tyrosine 551 (Y(551)). However, the detailed molecular mechanism regulating Btk activation remains unclear. In particular, we do not fully understand the correlation of kinase activity with Y(551) phosphorylation, and the role of the noncatalytic domains of Btk in the activation process. Insect cell expressed full-length Btk is enzymatically active, but a truncated version of Btk, composed of only the kinase catalytic domain, is largely inactive. Further characterization of both forms of Btk by mass spectrometry showed partial phosphorylation of Y(551) of the full-length enzyme and none of the truncated kinase domain. To determine whether the lack of activity of the kinase domain was due to the absence of Y(551) phosphorylation, we developed an in vitro method to generate Y(551) monophosphorylated Btk kinase domain fragment using the Src family kinase Lyn. Detailed kinetic analyses demonstrated that the in vitro phosphorylated Btk kinase domain has a similar activity as the full-length enzyme while the unphosphorylated kinase domain has a very low k(cat) and is largely inactive. A divalent magnesium metal dependence study established that Btk requires a second magnesium ion for activity. Furthermore, our analysis revealed significant differences in the second metal-binding site among the kinase domain and the full-length enzyme that likely account for the difference in their catalytic profile. Taken together, our study provides important mechanistic insights into Btk kinase activity and phosphorylation-mediated regulation.

Characterization of the interactions between the active site of a protein tyrosine kinase and a divalent metal activator

BACKGROUND

Protein tyrosine kinases are important enzymes for cell signalling and key targets for anticancer drug discovery. The catalytic mechanisms of protein tyrosine kinase-catalysed phosphorylation are not fully understood. Protein tyrosine kinase Csk requires two Mg2+ cations for activity: one (M1) binds to ATP, and the other (M2) acts as an essential activator.

RESULTS

Experiments in this communication characterize the interaction between M2 and Csk. Csk activity is sensitive to pH in the range of 6 to 7. Kinetic characterization indicates that the sensitivity is not due to altered substrate binding, but caused by the sensitivity of M2 binding to pH. Several residues in the active site with potential of binding M2 are mutated and the effect on metal activation studied. An active mutant of Asn319 is generated, and this mutation does not alter the metal binding characteristics. Mutations of Glu236 or Asp332 abolish the kinase activity, precluding a positive or negative conclusion on their role in M2 coordination. Finally, the ability of divalent metal cations to activate Csk correlates to a combination of ionic radius and the coordination number.

CONCLUSION

These studies demonstrate that M2 binding to Csk is sensitive to pH, which is mainly responsible for Csk activity change in the acidic arm of the pH response curve. They also demonstrate critical differences in the metal activator coordination sphere in protein tyrosine kinase Csk and a protein Ser/Thr kinase, the cAMP-dependent protein kinase. They shed light on the physical interactions between a protein tyrosine kinase and a divalent metal activator.

Modulation of cyclin-dependent kinase 4 by binding of magnesium (II) and manganese (II)

All kinases require an essential divalent metal for their activity. In this study, we investigated the metal dependence of cyclin-dependent kinase 4 (CDK4). With Mg(2+) as the essential metal and MgATP being the variable substrate, the maximum velocity, V, was not affected by changes in metal concentration, whereas V/K was perturbed, indicating that the metal effects were mainly derived from a change in the K(m) for MgATP. Analysis of the metal dependence of initial rates according to a simple metal binding model indicated the presence on enzyme of one activating metal-binding site with a dissociation constant, K(d(a)), of 5 +/-1 mM, and three inhibitory metal-binding sites with an averaged dissociation constant, K(d(i)), of 12+/-1 mM and that the binding of metal to the activating and inhibitory sites appeared to be ordered with binding of metal to the activating site first. Substitution of Mn(2+) for Mg(2+) yielded similar metal dependence kinetics with a value of 1.0+/-0.1 and 4.7+/-0.1 for K(d(a)) and K(d(i)), respectively. The inhibition constants for the inhibition of CDK4 by MgADP and a small molecule inhibitor were also perturbed by Mg(2+). K(d(a)) values estimated from the metal variation of the inhibition of CDK4 by MgADP (6+/-3 mM) and a small molecule inhibitor (3+/-1 mM), were in good agreement with the K(d(a)) value (5+/-1 mM) obtained from the metal variation of the initial rate of CDK4. By using the van't Hoff plot, the temperature dependence of K(d(a)) and K(d(i)) yielded an enthalpy of -6.0 +/- 1.1 kcal/mol for binding of Mg(2+) to the activating site and -3.2 +/- 0.6 kcal/mol for Mg(2+) binding to the inhibitory sites. The values of associated entropy were also negative, indicating that these metal binding reactions were entirely enthalpy-driven. These data were consistent with metal binding to multiple sites on CDK4 that perturbs the enzyme structure, modulates the enzyme activity, and alters the affinities of inhibitor for the metal-bound enzyme species. However, the affinities of small molecule inhibitors for CDK4 were not affected by the change of metal from Mg(2+) to Mn(2+), suggesting that the structures of enzyme-Mg(2+) and enzyme-Mn(2+) were similar.

Cryopreservation of the cyclic 3',5'-adenosine monophosphate-dependent protein kinase from bovine cardiac muscle

A method for the cryogenic storage of the cAMP-dependent protein kinase from bovine cardiac muscle is described. The catalytic parameters, kcat, KM, and kcat/KM are used to assess the activity of the enzyme both prior and subsequent to the freeze-thaw cycle. The enzyme is stored in cryogenic vials at -196 degrees C in liquid nitrogen. Complete retention of catalytic activity is dependent upon a rapid and efficient freeze-thaw cycle and the use of morpholinepropanesulfonic acid as the buffer. In addition, this buffer appears to eliminate the KCl- or NaCl-induced damage typically observed for enzymes stored at low temperature in phosphate buffer. As a result, morpholinepropanesulfonic acid may prove to be a more appropriate cryopreservation buffer than phosphate when the presence of salt is required for enzyme solubility or stability.

Intrasubstrate steric interactions in the active site control the specificity of the cAMP-dependent protein kinase

The cAMP-dependent protein kinase catalytic subunit phosphorylates serine residues more efficiently than threonine residues in synthetic peptides. In marked contrast, both amino acids are phosphorylated at similar rates when contained within the appropriate intact protein substrate. The structural basis for the discriminatory behavior observed in small peptides has been investigated and found to be a result of intrapeptide steric interactions in the vicinity of the threonine alcohol moiety. Leu-Arg-Arg-Gly-Thr-Leu-Gly, which is nearly free of these interactions, is phosphorylated at a rate that is almost comparable to its serine-containing counterpart.