Small Heat Shock Proteins from Bacteria and a Bacteriophage

Abstract

Proteins can unfold during heat and other types of stress and tend to aggregate with subsequent loss of function. This event is prevented by small Heat Shock Proteins (sHSPs) acting as molecular chaperones. sHSPs do not bind to native proteins but interact with unfolding proteins during stress conditions and transfer them to the other heat shock proteins for proper folding once normalcy is restored. sHSPs thus maintain cellular proteostasis and unlike the other heat shock proteins of higher molecular weight, sHSPs function in an ATP-independent manner. sHSPs are present in all kingdoms of life with many organisms having multiple sHSPs with little overall sequence identity. The molecular weight of sHSPs varies between 12 to 40 kDa. The structure of and#61537;-crystallin domain present in sHSPs is conserved and is flanked by flexible N- and C- terminal regions with roles mainly in substrate binding and oligomerization, respectively. sHSPs often form large oligomeric assemblies with the dimeric unit as the basic building block. sHSPs exist in a dynamic equilibrium by constant exchange of subunits between oligomers of various sizes. They have a broad substrate specificity, however, there is a dearth of knowledge on the exact mechanism of substrate binding and the necessity for the formation and exact functioning of different oligomers. The available structural information on sHSPs is limited as the three-dimensional structures of only a few higher order oligomers have been determined to date. To obtain insights into the functions and structures of sHSPs and the correlation between them and as a part of an ongoing project on sHSPs, we have carried out biophysical, biochemical and structural investigations on sHSPs of three bacteria: Bacillus cereus, Staphylococcus aureus and a cyanobacterium Synechococcus sp. WH7803; and a bacteriophage of Synechococcus sp. WH7803, called Synechococcus phage S-ShM2, which are presented in this thesis. The full length and a few deletion constructs of these four sHSPs were generated...