Understanding What Small Heat Shock Proteins Do for Bacterial Cells

Abstract

Being initially discovered due to the increased transcription of their encoding genes under heat shock conditions, heat shock proteins (HSPs, interchangeably named as ‘stress proteins’ or ‘molecular chaperones’) are known for their dramatic increase in amount when an organism is exposed to a variety of stress conditions. The major known activity for HSPs is to prevent the aggregation of nascent polypeptide chains that are yet-folded or mature proteins that are denatured under stress conditions. By transiently binding to and subsequently releasing the client proteins, HSPs function to promote their folding and/or assembly in living organisms. Small heat shock proteins (sHSPs), being relatively small among the HSPs in terms of the molecular weight of a single polypeptide chain, was found to present in bacteria only years after they were first identified in animals and plants. In this chapter, I will provide a historical perspective on what we have learned about the structure, function and regulation of sHSPs in bacteria. Main aspects covered in this chapter include the following. sHSPs exist as large dynamic homo-oligomers and regulate their activities by modulating their oligomeric status in a stress-responsive manner; sHSPs exhibit effective chaperone-like activities under in vitro and in vivo conditions; The monomeric small heat shock proteins possess an immunoglobulin-like folding pattern; sHSPs associate with and affect the physical state of cellular membranes; sHSPs play a potential role for bacterial cells to enter the non-growing dormant state; The biological functions of sHSPs are explored via gene knockout studies. In the end, I will also briefly discuss some of the unresolved issues regarding the structure, function and regulation of sHSPs.