Protein kinase A (PKA) activity was detected in haemocytes of the greater wax moth, Galleria mellonella larvae using kemptide, a specific peptide substrate. The enzyme was activated in vitro by 1μM concentration of cAMP, its analogs 8-Br-cAMP, 8-Chl-cAMP, as well as by BzcMP. Cyclic GMP was much less effective in haemocyte PKA activation. Incubation of G. mellonella haemocytes in vitro in the presence of a cell-permeable, specific PKA inhibitor, Rp-8-Br-cAMPS, induced changes in haemocyte morphology resembling those caused by live bacteria. Immune challenge of G. mellonella larvae with bacteria led to changes in haemocyte PKA activity. Gram-positive Micrococcus luteus was a better inducer of PKA activity than Gram-negative Escherichia coli. The kinetics of activity changes was dependent on the used bacteria and considerably differed from that observed in water-treated insects. Four potential PKA substrates of 155kDa, 44kDa, 40kDa and 22kDa were detected in haemocytes of naive larvae by phospho-motif antibodies recognizing PKA phosphorylation consensus site. The modification level of 40kDa protein changed in haemocytes of water- as well as both bacteria-treated G. mellonella larvae, whereas that of 155kDa protein changed only after E. coli injection. Additionally, in the haemocytes of bacteria-challenged insects a transient phosphorylation of 36kDa protein was detected.

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