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Aggregated and disaggregated forms of gizzard myosin rod and its fragments in various concentrations of NaCl (0-0.30 M) at various pH (7.4-8.6) were distinguished from each other by their permeability through a Sepharose 4B column. The rod existed in three forms, namely: large aggregates impermeable to the column, small aggregates eluted at the void volume of the column and a disaggregated monomer which penetrated the column. The relative proportions of the three forms varied depending on the salt concentration and pH. The monomeric rod was detected in NaCl solutions above 0.20 M and its relative proportion at 0.25 M NaCl was larger than those of the small and large aggregates. The small aggregates of the rod were predominant at below 0.05 M NaCl and, upon decrease in pH from 8.6 to 7.4, these small aggregates in NaCl solutions between 0.10 M and 0.15 M were replaced by the large aggregates. Light meromyosin, which corresponded to the C-terminal two-thirds of the rod, existed exclusively as large aggregates in NaCl solutions below 0.15 M; increase of NaCl concentration to above 0.20 M resulted in the formation of its monomer, instead of the large aggregates. In contrast to the rod, no small aggregated form of the light meromyosin was detected. Truncated light meromyosin which had lost a small segment from either the C-terminal or N-terminal of light meromyosin was eluted only as a monomer in any NaCl concentration at any pH. It may be deduced from the above results that a small segment in the light meromyosin is requisite for the assembly of both rod and light meromyosin in the NaCl solutions below 0.15 M and that the relative proportion of small and large aggregates of the rod is determined in a pH-dependent manner by the subfragment 2 segment, the N-terminal third of the rod. [ABSTRACT FROM AUTHOR]