Isoforms of cAMP-dependent protein kinase in the bivalve mollusk Mytilus galloprovincialis:: activation by cyclic nucleotides and effect of temperature

Two different isoforms of cAMP-dependent protein kinase (PKA) have been partially purified from the posterior adductor muscle and the mantle tissue of the sea mussel Mytilus galloprovincialis. The holoenzymes contain as regulatory subunit (R) the previously identified isoforms R-myt1 and R-myt2, and were named PKA(myt1) and PKA(myt2), respectively. Both cAMP and cGMP can activate these PKA isoforms completely, although they exhibit a sensitivity approximately 100-fold higher for cAMP than for cGMP. When compared to PKA(myt2), the affinity of PKmyt1 for cAMP and cGMP is 2- and 3.5-fold higher, respectively. The effect of temperature on the protein kinase activity of both PKA isoforms was examined. Temperature changes did not affect significantly the apparent activation constants (K-a) for cANIP. However, the protein kinase activity was clearly modified and a remarkable difference was observed between both PKA isoforms. PKA(myt1) showed a linear Arrhenius plot over the full range of temperature tested, with an activation energy of 15.3 +/- 1.5 kJ/mol. By contrast, PKA(myt2) showed a distinct break in the Arrhenius plot at 15 degreesC the activation energy when temperature was above 15 degreesC was 7-fold higher than that of lower temperatures (70.9 +/- 8.1 kJ/mol vs 10.6 +/- 6.5 kJ/mol). These data indicate that, above 15 degreesC, PKA(myt2) activity is much more temperature-dependent than that of PKA(myt1). This different behavior would be related to the different role that these isoforms may play in the tissues where they are located. (C) 2004 Elsevier Inc. All rights reserved.