Life as we know it requires water. Among other things, water hydrates the molecules that make up cells. When water is removed things go terribly wrong:membranes fuse, proteins aggregate and metabolic reactions slow or stop. However, some organisms, including some multicellular plants and animals can survive for long periods of time in the absence of cellular water. The organisms are called the anhydrobiotes.
Very little is known about the genetic requirements needed to survive anhydrobiosis, but there are a handful of genes that seem to be specific to organisms that can survive in the dry state. One family of genes that may be involved in desiccation tolerance codes for proteins known as late embryogenesis abundant or LEA proteins. LEA proteins, which tend to be unstructured in solution and are also extremely hydrophilic, have long been known to accumulate to very high levels in dehydration-tolerant plant seeds. More recently, LEA proteins have been identified and isolated in a number of invertebrates that exhibit tolerance of dehydration, including the nematode Aphelenchus avenae. Could LEA proteins be involved in stabilising some aspect of a cell's structure during dehydration? In their recent PNAS report, Sohini Chakrabortee and colleagues at the University of Cambridge and Cambridge Institute for Medical Research set out to test whether a nematode LEA protein could act to protect other proteins from aggregation during dehydration.
To test whether the nematode LEA proteins could prevent protein aggregation during dehydration, the team isolated water soluble proteins from nematode and human cells and subjected them to vacuum dehydration in the presence or absence of purified nematode LEA proteins. They also added the LEA proteins at different times before and during the dehydration–rehydration process. They found that the LEA proteins prevent aggregation of both nematode and human proteins when used at a ratio of 5:1 or greater. This effect was observed in all size classes of proteins, and thus it appears that LEA proteins act in a general manner, and not on a specific subset of proteins. The greatest protection from aggregation was gained when the LEA proteins were added before dehydration, but addition of LEA proteins after dehydration and within 5 min of rehydration did afford some protection.
The team also tested the ability of the nematode LEA protein to prevent aggregation of proteins under hydrated conditions. To do this, they created human cell lines that co-expressed a huntingtin-derived protein (that is prone to aggregation) and the nematode LEA protein. They found that the LEA protein significantly reduced the number of cells that accumulated huntingtin-derived protein aggregates.
Chakrabortee and colleagues have illustrated that the nematode LEA protein functions in general to prevent protein aggregation, but is it acting as a molecular chaperone or a shield? The fact that the LEA proteins are functional while remaining highly unstructured seems to suggest that the LEA proteins act as molecular shields by coating proteins and preventing them from aggregating,rather than through a conformation-dependent mechanism as would be expected for a molecular chaperone. However, a definitive answer to this question remains to be tested.