The Ca 2+ -induced transition in the troponin complex (Tn) regulates vertebrate striated muscle contraction. Tn was reconstituted with recombinant forms of troponin I (TnI) containing a single intrinsic 5-hydroxytryptophan (5HW). Fluorescence analysis of these mutants of TnI demonstrate that the regions in TnI that respond to Ca 2+ binding to the regulatoryN-domain of TnC are the inhibitoryregion (residues 96–116) and a neighboring region that includes position 121. Our data confirms the role of TnI as a modulator of the Ca 2+ affinityof TnC; we show that point mutations and incorporation of 5HWinTnI can affect both the affinityand the cooperativityof Ca 2+ binding to TnC
