Structural Properties of Silkworm Small Heat-Shock Proteins: sHSP19.9 and sHSP20.8

sHSP20.8 and sHSP19.9 are silkworm small-heat shock proteins (sHSPs) comprising a number of polypeptides of molecular sizes of several tens of kilodaltons as subunits. The structural properties of sHSPs were investigated. sHSP19.9 was found to be aggregated by itself during incubation at 60 °C. Aggregation was suppressed in the presence of dithiothreitol and at high ionic strength. In contrast, sHSP20.8 was not aggregated. Aggregation of sHSP19.9 was partially suppressed by sHSP20.8 and in the presence of catalase as a target protein. Based on changes in small-angle X-ray scattering, it is possible that the molecular size of sHSP19.9 is larger than that of sHSP20.8, and that their molecular sizes increase with increasing temperature in a reversible, biphasic manner. sHSPs did not protect catalase from thermal inactivation, but protected it from precipitation by forming a soluble complex. sHSP20.8 and sHSP19.9 with dithiothreitol were stable against lyophilization, autoclaving at 120 °C, and boiling.