The proteolytic susceptibility of specific sites in myosin light chains is modulated by the filament conformation

The proteolytic susceptibilities of specific sites in the LC1 and LC2 N-termini were modulated by ionic strength in myosin (a species able to form filaments) but not in S1. (a) In the presence of Ca2+ or Mg2+, the proteolytic susceptibility (apparent initial reaction rate) showed a sharp discontinuity at a critical ionic concentration similar for LC1', LC2' and LC2'' cleavages. (b) The susceptibility of LC1' and LC2'' was higher at low ionic concentration in the more compact structure of the filament than in the dissociated form at high ionic concentration. (c) The ionic concentration effect was no longer observed with species unable to form filaments. (d) This effect occurred at a critical ionic concentration markedly different from the critical concentration at which the monomer-filament equilibrium was found. These observations lead to the following conclusions. (a) The ionic concentration effect is an attribute of the filament structure. (b) In the filament the faster cleavage at sites (LC1' and LC2'') near the LC1 and LC2 N-termini are due to an extended configuration of the N-terminal segment binding to a site in the filament structure. (c) The slower rate of formation of LC2' in the filament indicates that the N-terminal segment of LC2 binds more tightly to the structure than that of LC1. (d) The critical ionic concentration is not that of the filament-monomer equilibrium but corresponds to the order-disorder transition of the heads in the filament. These results suggest that the N-termini of the light chains (here in striated muscles) play a role in a secondary regulatory mechanism. The analysis of these regions may contribute to our understanding of the altered activity and regulation seen in such diseases as idiopathic dilated cardiomyopathy [Margossian, S. S., White, H. D., Caulfield, J. B., Norton, P., Taylor, S. & Slayter, H. S. (1992) Circulation 85, 1720-1733].