|| Checking for direct PDF access through Ovid
The tandem GAF domain of human phosphodiesterase 11A4 (hPDE11A4) requires 72 μM cGMP for half-maximal effective concentration (EC50) of a cyanobacterial adenylyl cyclase used as a reporter enzyme. Here we examine whether modifications in the N-terminus of PDE11A4 affect cGMP signalling. The N-terminus has two phosphorylation sites for cyclic nucleotide monophosphate-dependent protein kinases (Ser117, Ser168). Phosphorylation of both by cAMP-dependent protein kinase decreased the EC50 value for cGMP from 72 to 23 μM. Phosphomimetic point mutations (S117D/S167D), which project complete phosphorylation, lowered the EC50 value to 16 μM. Structural and sequence data indicate that 196 amino acids precede the start of the GAF domain in hPDE11A4. Removal of 197 amino acids yielded unregulated cyclase activity, whereas truncation by 196 amino acids resulted in a cGMP-regulated protein with a cGMP EC50 value of 7.6 μM. Truncation by 176 amino acids was required for cGMP EC50 values to decrease to below 10 μM; a construct truncated by 168 amino acids had an EC50 value of 224 μM. The decrease in EC50 values was accompanied by a sixfold increase in basal activity; the extent of cGMP stimulation remained unaffected, however. We conclude that N-terminal modifications strongly affect cGMP regulation of hPDE11A4.