Using ion-exchange chromatography, we have identified and isolated two forms of a cyclic GMP-specific binding activity in filter-broken cell extracts of Dictyostelium discoideum. Upon addition of excess cold ligand, one form (S-type) released bound 3H-labelled cyclic GMP very slowly (t½ ≈ 68 min), while the other form (F-type) released the cyclic GMP in <1 min. After photoaffinity labelling with 32P-labelled cyclic GMP, both forms revealed a major 160x103Mr band (and a few bands of lower molecular weight) on autoradiograms of sodium dodecyl sulphate-polyacrylamide gels. Addition of 500 mM-NaCl to S-type activity converted the activity to a fast-dissociating form indistinguishable from F-type, and this conversion was reversed by dialysis. Salt treatment or dialysis had no appreciable effect on the association/dissociation kinetics of F-type activity. When crude S-type activity was heated (to destroy cyclic GMP binding) and then added to F-type activity, the latter activity acquired slow-dissociating properties identical to S-type. This result suggested that the cells possess a ‘factor’ that can dramatically alter binding properties of this cyclic GMP-binding protein. Crude preparations of this factor were by boiling or proteases, but were sensitive to RNase A. Further studies revealed that acids (in particular, DNA) could effectively mimic the factor in its ability to modulate the binding kinetics of the cyclic GMP-binding activity.

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