ABERRANT CGMP-BINDING ACTIVITY IN NON-CHEMOTACTIC DICTYOSTELIUM-DISCOIDEUM MUTANTS

The kinetics of cGMP-binding to the major cGMP-binding activity in Dic tyostelium, were investigated in 10 non-chemotactic mutants (KI mutant s; KI-1 similar to 10). A wild-type cell contains about 3000 binding s ites with a K-d of 1.5 nM. cGMP may dissociate from these binding site s with fast (F-type) or slow (S-type) kinetics, and DNA has been shown to promote the conversion of F- to S-type of cGMP-binding. The 10 mut ants were placed in 4 classes, based on equilibrium and non-equilibriu m binding properties and the effect of DNA. Class I mutants (KI-I, 3 a nd 8) have normal cGMP-binding properties. Class II mutants (KI-2, 6 a nd 7) show increased K-d values but nearly normal B-max, normal F/S ra tio and normal effects of DNA. Class III mutants (KI-4, 5 and 10) have a strongly decreased K-d and increased B-max, nearly all binding site s are of the S-type and DNA does not affect the binding; apparently th ese mutants have a cGMP-binding protein locked in the S-form. cGMP-bin ding in class IV mutant (KI-9) is normal except that the number of bin ding sites is increased about S-fold. The finding of seven mutants wit h altered cGMP-binding in 10 non-chemotactic mutants suggests that the cGMP-binding activity plays an important role in the chemotactic sign al transduction pathway.