FC0078
Anhydrin  -  DNA

Biological function
Anhydrin’s gene is induced in response to dehydration in the anhydrobiotic nematode, Aphelenchus avenae. Anhydrin is entirely intrinsically disordered yet possesses two apparently independent functions: a chaperone-like molecular shield, which is able to reduce aggregation of client proteins in vitro and in the nucleus, and a novel endonuclease.

Structural evidence
Interaction between anhydrin and supercoiled plasmid DNA, pcDNA3.1 was tested by NMR. 1D ¹H-NMR spectrum indicated binding of a significant proportion of anhydrin molecules to the plasmid which, because of its size, has slow diffusion properties leading to broadening of amide resonances in the complexed protein so they are no longer detected. NMR experiments indicate binding of anhydrin to DNA, both CD and FTIR analysis showed no increase in folding in the presence of DNA and thus anhydrin remains disordered.

Biochemical evidence
With increasing concentration of anhydrin, the supercoiled (SC) form of the plasmid gradually disappeared, being replaced by open-circular (OC) plasmid and an additional band that migrated between them, which was later confirmed to be a linearized plasmid. Anhydrin also cleaves the plasmids pHM6 and pET28a+ showing that it recognizes a range of substrates, including linear DNA. The pattern of anhydrin endonuclease activity was compared with that of various known DNA modifying enzymes, but the profiles were not identical indicating a different mechanism of digestion. So anhydrin convert supercoiled plasmid DNA to linear form, probably via a nicked, open circle, and to digest linear DNA to small fragments; it also cleaves linker DNA in chromatin.

Mechanism category
tethering

Significance
Nuclease function for anhydrin would be of value in anhydrobiosis where desiccation is expected to cause DNA breakage, tangling, and stalled replication forks, resulting in structures that require resolution, repair, or removal during the rehydration phase. An alternative role might be in apoptosis or necrosis, where DNA breakdown is required. The precise structural role of fuzziness in making a dynamic assembly with DNA is unknown.