In Vitro Holdase Activity of E. coli Small Heat-Shock Proteins IbpA, IbpB and IbpAB: A Biophysical Study with Some Unconventional Techniques

E. coli small heat shock proteins IbpA and IbpB (inclusion body binding proteins A and B) are known to act as holding chaperones on denaturing, aggregate-prone proteins. But, there is no clear understanding about which of the IbpA and IbpB has more holdase activity and how the holdase activity of one was influenced by the presence of the other. This study was conducted to resolve the questions, using some uncommon physical techniques like dynamic light scattering, micro-viscometry and atomic force microscopy in addition to the common techniques of spectrophotometry and spectrofluorimetry. The holdase activity was investigated on the heat-denatured L-lactate dehydrogenase (LDH) of rabbit muscle. LDH was found to be deactivated completely without any aggregation at 52°C and with transient aggregation at 60°C; molecular dynamics simulation also revealed that at 52°C, denaturation occurred only at the active site of LDH. When LDH was allowed to be deactivated in the presence of IbpA, IbpB or (IbpA + IbpB), partial inhibition of i) denaturation at 52°C and ii) aggregation at 60°C were observed. The results further demonstrated that the holdase activity of IbpB was higher than that of IbpA and their combined effect was higher than their individual one.