Changes in the thermal unfolding of p-phenylenedimaleimide-modified myosinsubfragment 1 induced by its 'weak' binding to F-actin

Differential scanning calorimetry (DSC) was used to analyze the thermal unf olding of myosin subfragment 1 (S1) with the SH1 (Cys-707) and SH2 (Cys-697 ) groups cross-linked by N',N'-p-phenylenedimaleimide (pPDM-S1). It has bee n shown that F-actin affects the thermal unfolding of pPDM-S1 only at very low ionic strength, when some part of pPDM-S1 binds weakly to F-actin, but not at higher ionic strength (200 mM KCl), The weak binding of pPDM-S1 to F -actin shifted the thermal transition of pPDRI-S1 by about 5 degreesC to a higher temperature, This actin-induced increase in thermal stability of pPD M-S1 was similar to that observed with 'strong' binding of unmodified S1 to F-actin, Our results show that actin-induced structural changes revealed b y DSC in the myosin head occur not only upon strong binding but also on wea k binding of the head to F-actin, thus suggesting that these changes may oc cur before the power-stroke and play an important role in the motor functio n of the head. (C) 2001 Federation of European Biochemical Societies. Publi shed by Elsevier Science B.V. All rights reserved.