A study of paramyosin aggregation using transient electric birefringence techniques.

Abstract

AbstractThe aggregation of several forms of the molluscan muscle protein paramyosin at low concentration and at low ionic strength was studied in the pH range 6–10 using transient electric birefringence techniques. In the lower part of this pH range, the aggregates exhibit negative birefringence, but this changes to positive birefringence when the pH increases. Analysis of the field‐free birefringence decay transients of the paramyosin solutions showed that all aggregates coexisted with paramyosin monomer and allowed a determination of the rotational diffusion constant of the aggregates present at each pH. The size and shape of these aggregates were estimated from their rotational diffusion constants and were compared with the known characteristics of larger aggregates such as paracrystals. The positively birefringent aggregates appear to be staggered dimers at certain values of pH; at other pH values, these aggregates appear to be higher aggregates, probably formed by lateral aggregation of monomer or dimer onto one of these staggered dimers. The staggered dimers are formed by overlap of 200–600 Å along each cylindrical paramyosin molecule, in agreement with the 530‐Å overlap distance found in paramyosin paracrystals by Cohen et al. (1971) J. Mol. Biol. 56, 223–237. Some speculations are presented about the nature of the negatively birefringent species.