Dr. Hiromi Kagawa

December 21, 2006

“Biochemical Adaptations that Set the Physical and Chemical Limits for Life”

NCC 2–1338

It is a fundamental goal of astrobiology to elucidate the limits to which life has adapted on Earth in an effort to shed light on the inhabitability of worlds beyond Earth. All organisms are adapted to their environments and since all environments vary with time, all organisms have by necessity evolved biochemical strategies for coping with the changes they encounter within their environments. Our research efforts are focussed on understanding the biochemical adaptations that set the tolerable limits for the inevitable environmental variation with which an organism must cope. In particular, we are interested in understanding the strategies of organisms that appear to be ‘pushing the envelope’ for life by living in extremely harsh or extremely variable environments. How do organisms living in near boiling sulfuric acid (so-called hyperthermophilic acido-philes) adapt their biomolecules and their biochemical systems to thrive under these conditions?

One adaptation that nearly all organisms have made to stressful conditions is the production of specialized proteins, known as “stress proteins” or “chaperonins.” The chaperonins are a class of proteins that are thought to play a role in protein refolding, but our research over the last three years clearly demonstrates that these proteins are also playing a role in membrane stabilization.

The chaperonins in the hyperthermophile Sulfolobus shibatae are among the most abundant proteins, constituting as much as 12% of their total protein. These 60 kDa proteins are isolated from cells as double-ring structures. We have observed that purified chaperonins from S. shibatae form ordered filaments and proposed that these filamentous structures, rather than rings, are functional in vivo. Our previous research using immunofluorescence light microscopy (IFM) and immunogold electron microscopy (IEM) indicate that chaperonins are localized to the membrane. This finding is corroborated by centrifugation experiments, which indicate that as much as 90% of the chaperonin protein co-sediments with the membrane rather than the cytosolic fractions. On the basis of these observations, we hypothesized that the role of chaperonins, or chaperonin filaments, in vivo is to stabilize and help regulate the fluidity of the cytoplasmic membrane under normal and stress conditions.

We propose to further test this hypothesis and to clarify the structure and function of hyperthermophilic HSP60s. Our proposed goals are (1) to elucidate critical structural elements of the proteins that allow them to assume higher-order structures (rings and filaments) and (2) to confirm how the three different protein components of the HSP60s interact to define the functional unit in the living cell. To approach both of these goals we intend to use a genetic approach, i.e., to make mutants and to study them in vitro and in vivo. Our ultimate aim is to understand the role of HSP60/chaperonins in the physiology and molecular biology of hyperthermophiles and other organisms.