In my lab at NASA Ames, we do cultivate some extremophilic single-celled microbes; not MONSTERS. One such extremophile, called Sulfolobus Shibatae, comes from Japan and lives at 85-degrees Celsius (185 degree Fahrenheit) and pH 2 — that’s near-boiling sulfuric acid. We study a class of proteins these organisms make called heat shock proteins (HSBs). It’s been known for a long time that one of the ways that organisms adapt to high temperatures in their environment is by making heat shock proteins. Among the five classes of known HSPs, the HSP60, called HSP60 because of the molecular mass of the proteins in this class, are among the most highly conserved. This means that all the known HSP60s have very similar amino acid sequences. Sulfolobus makes a lot of its HSP60s, particularly when it’s grown at near-lethal high temperatures. This is interesting in itself because most other organisms make many different HSPs when they are exposed to high temperatures, but Sulfolobus makes only HSP60. Even more interesting is that these Sulfolobus HSP60s are not conserved with other HSP60s. The Sulfolobus HSP60s are very closely related to a protein in mice and human beings known as TCP1, which wasn’t previously known to be related to HSP60s. I discovered this in 1991 and published it in the science journal Nature. At the time, I concluded that the Sulfolobus HSP60 was like other HSP60s and since my colleagues thought the function of all HSP60s is to play a role in folding other proteins, I suggested in that Nature paper that the Sulfolobus HSP60 and TCP1 play a role in protein folding. I now know that this hypothesis is wrong. I know this because in Sulfolobus the HSP60s or their chaperonin-like structures, which I call “rosettasomes,” are associated with the cell membrane and that’s not where most protein folding occurs in the cell.
NTB: What are Rosettasomes, and what role do they play in the life process?
Dr. Trent: In Sulfolobus, 18 individual HSP60 molecules get together to form double-rings with 9 HSP60s per ring. I discovered these double ring structures and named them “rosettasomes” to describe their “rosette” shape; “somes” means bodies—they are rosette-shaped bodies. I’ve come to the conclusion that their role is not protein folding, but to help stabilize the cell membrane. I discovered this by locating rosettasomes in Sulfolobus cells and found that under all growth conditions they are associated with the cell membrane. Sulfolobus lives in acid, but it doesn’t have acid inside its cell. However, when Sulfolobus is stressed at high temperatures the cells leak. Sulfolobus responds to heat by making a lot of rosettasomes and they are associated with the cell membrane. We then observed that when Sulfolobus has a lot of rosettasomes on its membrane it doesn’t leak as much. Rosettasomes are doing something to prevent the membranes from leaking, but we don’t know what that is yet. We haven’t yet figured out how rosettasomes stabilize the cell membrane against leakage, but we’re working on it.