The critical concentration required for filament assembly in vitro from highly 
purified desmin was determined by both turbidity and centrifugation assays. 
Assembly was done in the presence of 2 mM-Ca2+, 2 mM-Mg2+ or 150 mM-Na+ at 2, 22 
and 37 degrees C. Similar values for critical concentration were obtained by 
both assays. As temperature increased, critical concentration decreased for each 
cation. The critical concentration was lowest in the presence of Ca2+ at 2, 22 
and 37 degrees C, but was highest in the presence of 150 mM-Na+ at 2 degrees C. 
Negative staining showed that supernatants from the centrifugation assays 
contained protofilaments, protofibrils and short particles (less than 300 nm), 
but pellets contained long filaments (greater than 1 micron) with an average 
diameter of 10 nm. As the temperature increased, both the average diameter and 
average length of particles in the supernatant increased. Thermodynamic analysis 
indicated that hydrophobic interactions were dominant during desmin assembly, 
but that ionic interactions might also be involved. Our results demonstrated 
that the specific cation and temperature and temperature-cation interactions all 
are important in assembly of desmin intermediate filaments.