Functional Distribution of Archaeal Chaperonins

Abstract

Chaperonin, also known as heat shock protein 60 (Hsp60), belongs to an evolutionarily conserved protein family that enables cells to survive under stressful conditions, including elevated or reduced temperature, high or low salinity, acidic/alkaline environments, and high or low osmotic pressure. Archaeal genomes generally contain multiple genes encoding chaperonins. For example, the hyperthermophilic archaeon Thermococcus kodakarensis, which grows optimally at 85 °C, has both a cold-inducible (CpkA) and a heat-inducible (CpkB) chaperonin, which are involved in adaptation to low and high temperatures, respectively. These two chaperonins share high sequence identity (77%), except in their carboxy-terminal regions. Furthermore, depletion of cpkA or cpkB results in growth defects under cold stress (60 °C) or heat stress (93 °C), respectively, but not at the optimal temperature (85 °C). These observations indicate that CpkA and CpkB are necessary for cell growth at lower and higher stressed temperatures, respectively. Immunoprecipitation studies using specific antisera revealed that CpkA and CpkB recognize different types of proteins, i.e., that they have distinct substrate spectra. Likewise, several extremophilic archaea encode paralogous chaperonins that are differentially regulated during stresses such as heat, cold, high salt, pH, pressure, and nutrient deprivation, suggesting that these chaperonins might encounter different substrates depending on the type of stress confronting the cell. Extra chaperonin genes may have arisen to assist in protein folding under types of selective pressure in which the constitutively expressed chaperonin is unable to function, and here we speculate about the significance of multiple chaperonin genes in archaea.