The interaction of troponin C with phosphofructokinase. Comparison with calmodulin.

Phosphofructokinase (PFK) is a calmodulin (CaM)-binding protein [Mayr & Heilmeyer (1983) FEBS Lett. 195, 51-57]. We found that troponin C (TnC), which is homologous to CaM, also binds PFK and affects PFK's catalytic activity, aggregation states and conformational changes as CaM does in most cases. PFK titration of N-acetylaminoethyl-5-naphthylamido-1-sulphonate ('AEDANS')-TnC showed that its apparent dissociation constant is comparable with that of PFK-CaM. Fluorescent labels were also used to probe contact regions on TnC and CaM. It is likely that the C-terminal end of the connecting strand of the TnC molecule is close to PFK in the binary complex. Hydrophobic regions of TnC and CaM also possibly play roles in the binding and polymerization of PFK. TnC and CaM deactivate PFK through accelerating PFK conformational change as well as through accelerating PFK tetramer dissociation, as implied in the results of activity, light-scattering, fluorescence and c.d. experiments. The intact molecule of CaM appears to be required to deactivate PFK, because neither half of the CaM molecule has an effect on PFK activity.