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 t o 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 L C1', 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 concentrati on. (c) The ionic concentration effect was no longer observed with spe cies 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 observatio ns lead to the following conclusions. (a) The ionic concentration effe ct 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-termi ni are due to an extended configuration of the N-terminal segment bind ing to a site in the filament structure. (c) The slower rate of format ion of LC2' in the filament indicates that the N-terminal segment of L C2 binds more tightly to the structure than that of LC1. (d) The criti cal ionic concentration is not that of the filament - monomer equilibr ium but corresponds to the order-disorder transition of the heads in t he filament. These results suggest that the N-termini of the light cha ins (here in striated muscles) play a role in a secondary regulatory m echanism. The analysis of these regions may contribute to our understa nding of the altered activity and regulation seen in such diseases as idiopathic dilated cardiomyopathy [Margossian, S. S., White, H. D., Ca ulfield, J. B., Norton, R, Taylor, S. & Slayter, H. S. (1992) Circulat ion 85, 1720-1733].