Mechanism of activation of protein kinase I from rabbit skeletal muscle. Investigation with agarose-immobilized cAMP derivatives

The synthesis and characterisation of a cyclic AMP-Sepharose 4B matrix with affinity for cyclic AMP-dependent proteins

A novel synthesis route for the preparation of a cyclic AMP-containing affinity adsorbent is described. This involves the use of a simple single-vessel, two-step, solid-phase reaction in which 5'-AMP, originally coupled to the matrix in the form of 5'-AMP-Sepharose 4B, is cyclised by a sequence involving activation of the phosphate moiety, followed by intramolecular condensation to give a 3',5'-phosphodiester. Physicochemical evidence, including 31P NMR studies, shows that this cyclisation takes place leaving no trace of original phosphomonoesters. The gel retains its excellent chromatographic properties, but now its specificity is directed towards cAMP-dependent proteins. Protein kinase (EC 2.7.1.37) binds to the gel, the catalytic subunit only being eluted when a simple buffer 0.2 M phosphate/1 mM theophylline is applied to column, whilst the regulatory unit is eluted using 20% ethylene glycol of 2% Pharmalyte, pH 5-8. This gel should find a use in the purification of protein kinase subunits and, by inference, of other cAMP-dependent proteins.

A model for the chemical interactions of adenosine 3':5'-monophosphate with the R subunit of protein kinase type I. Refinement of the cyclic phosphate binding moiety of protein kinase type I

The cAMP receptor site in the regulatory subunit of adenosine 3':5'-monophosphate (cAMP)-dependent protein kinase type I was mapped using analogues of cAMP in which the ribose phosphate moiety was systematically modified. Electronical alteration of the cyclophosphate ring at the 3' and 5' positions by sulfur and nitrogen decreased the affinity of these analogues towards the kinase. Substituents at these positions are not tolerated. Testing the separated diastereomers of derivatives in which one of the exocyclic oxygens at the phosphorus has been substituted by sulfur, it was found that one diastereoisomer is preferentially recognized. Based on these results it is proposed that the hydrophylic cyclic phosphate-ribose moiety of cAMP is bound to the kinase via its 3' and 5'-oxygens, the 2'-hydroxy group and the negative charge in a fixed position. Based on our and other published results it is further proposed, that the adenine moiety is bound in a hydrophobic cleft without any hydrogen bond interactions. The chemical interactions between cAMP and the R subunit of protein kinase type I differ from those found for the binding of cAMP to the chemoreceptor of Dictyostelium discoideum [18].

Mechanism of activation of protein kinase I from rabbit skeletal muscle. The equilibrium parameters of ligand interaction and protein dissociation

Protein kinase I from rabbit skeletal muscle binds adenosine 3':5'-monophosphate (cAMP) and ATP with high affinity; cAMP promotes and ATP retards dissociation (activation) of the tetrameric enzyme. The interrelationship of ligand interaction with protein dissociation has been probed by quantitative ligand binding, using the filter assay technique, and by computer simulation of binding curves in terms of Scatchard plots. A comparison of the experimental and computed binding data strongly confirms the supposition that the dimeric regulatory subunit (R2) binds cAMP cooperatively. Mainly from ATP binding it is further concluded that the interaction of the two catalytic subunits with R2 is also strongly cooperative, whereas the binding of ATP to the holoenzyme is non-cooperative. The underlying random model, possessing low-affinity and high-affinity sites not only for ATP but also for cAMP, allows the determination of a minimal set of equilibrium parameters required for description and also an estimation of their magnitude. It further provides a basis for an explanation of the different behaviour of protein kinases I and II reported in the literature.

Mechanism of activation of protein kinase I from rabbit skeletal muscle. Mapping of the cAMP site by spin-labeled cyclic nucleotides

Binding of adenosine 3':5'-monophosphate (cAMP) to protein kinase (type I) from rabbit skeletal muscle has been investigated using spin-labeled cAMP derivatives. Different compounds were synthesized with the spin label attached by spacer chains of different length at different positions on the adenine base. Immobilization of the spin label, determined by comparing the electron-spin resonance spectra recorded in the presence of the kinase with those of the free ligand in solutions of different viscosities, gave information about the geometry of the cAMP site. Strong immobilization of the N-6 substituents up to a spacer length of seven atoms indicates a rather deep cleft of the cAMP site. The depth of this cleft differs, however, when the spin label is attached to the different positions at the adenine (N-6, C-2 and C-8). Whereas the N-6 derivatives indicate a rather deep site, the C-2 derivatives reveal a significantly smaller depth and C-8 substituents (syn conformation) obviously occupy a very shallow surface with almost no immobilation. In addition the binding affinities of the spin-labeled cAMP derivatives have been determined, together with those of a series of (diamagnetic) C-2 derivatives bearing hydrophobic alkyl chains of different length. The latter results helped to clarify the differences between the regions near to C-2 and N-6, respectively, of the cAMP site. N-6 spin-labeled derivatives have also been investigated in the presence of ATP and protein kinase. These results are interpreted as indicative of a conformational change at the cAMP site upon formation of the holoenzyme, due to binding of ATP, leaving cAMP less strongly immobilized.