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 Rmyt1 and Rmyt2, and were named PKAmyt1 and PKAmyt2, 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 PKAmyt2, the affinity of PKAmyt1 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 (Ka) for cAMP. However, the protein kinase activity was clearly modified and a remarkable difference was observed between both PKA isoforms. PKAmyt1 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, PKAmyt2 showed a distinct break in the Arrhenius plot at 15 °C; the activation energy when temperature was above 15 °C 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 °C, PKAmyt2 activity is much more temperature-dependent than that of PKAmyt1. This different behavior would be related to the different role that these isoforms may play in the tissues where they are located.