FLUORESCENT-PROBES ATTACHED TO CYS-35 OR CYS-84 IN CARDIAC TROPONIN-CARE DIFFERENTIALLY SENSITIVE TO CA2-DEPENDENT EVENTS IN-VITRO AND IN-SITU()

The goal of the current study was to generate recombinant cTnC protein s with single Cys residues as sites for attachment of fluorescent prob es that can distinguish between the structural effects of myosin cross bridges and direct Ca2+ binding to cTnC (cardiac and slow skeletal tr oponin C) in skinned fibers. We anticipated that cTnC proteins which r etain the endogenous Cys 35 (cTnC(C35)) or Cys 84 (cTnC(C84)) would pr ovide fluorescent probes with distinct microenvironments, since these residues are on opposite sides of the globular regulatory domain. In v itro experiments that showed IAANS -(4'-(iodoacetamido)anilino)naphtha lene-6-sulfonic acid) coupled to Cys 35 can induce unwanted structural perturbations as evidenced by a decreased affinity of site II for Ca2 + when IAANS-labeled cTnC(C35) is bound to cTnI. Important structural features involving Cys 35 in the inactive site I are suggested by a Ca 2+-dependent increase in reactivity of Cys 35 with sulfhydryl specific reagents when cTnC(C35) is associated with cTnI. These characteristic s are not seen for cTnC(C84). When incorporated in situ into skinned c ardiac muscle fibers, native cTnC with IAANS bound to both Cys 35 and Cys 84 showed a pCa(50) of fluorescence which preceded that of force, while the pCa(50) values of both force and fluorescence were coinciden t for IAANS-labeled cTnC(C84). Disruption of force-producing myosin cr oss bridges had no effect on the pCa(50) of fluorescence for IAANS-lab eled cTnC(C84), but induced a rightward shift in the pCa(50) of fluore scence for IAANS-labeled native cTnC. These data can be interpreted to indicate that cTnC with IAANS bound to both Cys 35 and C84 senses eit her myosin cross bridges or direct Ca2+ binding and myosin-induced coo perativity, while IAANS bound to Cys 84 alone senses conformations tha t are tightly coupled with force generation.