Catalytic and chaperone-like functions in an intrinsically disordered protein associated with desiccation tolerance.

Intrinsically disordered proteins (IDPs) lack well-defined structure but are widely represented in eukaryotic proteomes. Although the functions of most IDPs are not understood, some have been shown to have molecular recognition and/or regulatory roles where their disordered nature might be advantageous. Anhydrin is an uncharacterized IDP induced by dehydration in an anhydrobiotic nematode, Aphelenchus avenae. We show here that anhydrin is a moonlighting protein with two novel, independent functions relating to desiccation tolerance. First, it has a chaperone-like activity that can reduce desiccation-induced enzyme aggregation and inactivation in vitro. When expressed in a human cell line, anhydrin localizes to the nucleus and reduces the propensity of a polyalanine expansion protein associated with oculopharyngeal muscular dystrophy to form aggregates. This in vivo activity is distinguished by a loose association of anhydrin with its client protein, consistent with a role as a molecular shield. In addition, anhydrin exhibits a second function as an endonuclease whose substrates include supercoiled, linear, and chromatin linker DNA. This nuclease activity could be involved in either repair of desiccation-induced DNA damage incurred during anhydrobiosis or in apoptotic or necrotic processes, for example, but it is particularly unexpected for anhydrin because IDP functions defined to date anticorrelate with enzyme activity. Enzymes usually require precise three-dimensional positioning of residues at the active site, but our results suggest this need not be the case. Anhydrin therefore extends the range of IDP functional categories to include catalysis and highlights the potential for the discovery of new functions in disordered proteomes.

Trafficking of bdelloid rotifer late embryogenesis abundant proteins.

The bdelloid rotifer Adineta ricciae is an asexual microinvertebrate that can survive desiccation by entering an ametabolic state known as anhydrobiosis. Two late embryogenesis abundant (LEA) proteins, ArLEA1A and ArLEA1B, have been hypothesized to contribute to desiccation tolerance in these organisms, since in vitro assays suggest that ArLEA1A and ArLEA1B stabilize desiccation-sensitive proteins and membranes, respectively. To examine their functions in vivo, it is important to analyse the cellular distribution of the bdelloid LEA proteins. Bioinformatics predicted their translocation into the endoplasmic reticulum (ER) via an N-terminal ER translocation signal and persistence in the same compartment via a variant C-terminal retention signal sequence ATEL. We assessed the localization of LEA proteins in bdelloids and in a mammalian cell model. The function of the N-terminal sequence of ArLEA1A and ArLEA1B in mediating ER translocation was verified, but our data showed that, unlike classical ER-retention signals, ATEL allows progression from the ER to the Golgi and limited secretion of the proteins into the extracellular medium. These results suggest that the N-terminal ER translocation signal and C-terminal ATEL sequence act together to regulate the distribution of rotifer LEA proteins within intracellular vesicular compartments, as well as the extracellular space. We speculate that this mechanism allows a small number of LEA proteins to offer protection to a large number of desiccation-sensitive molecules and structures both inside and outside cells in the bdelloid rotifer.

Problems screening for HAND among the educationally disadvantaged.

Neurocognitive screeners are used to detect symptoms of HIV-Associated Neurocognitive Disorders (HAND). However, the degree to which education and socioeconomic status affect these screeners remains unclear. Neurocognitive screeners were administered to 187 socioeconomically disadvantaged HIV+ individuals upon entering treatment who had no other risk factors for HAND. The false positive rates were: 84% for the Montreal Cognitive Assessment, 59% for the International HIV Dementia Scale, and 28.3% for the Modified HIV Dementia Scale. Given these high false positive rates, the screeners may be more useful for establishing baseline functioning and sequential testing to detect deterioration.

Bioinformatics and protein expression analyses implicate LEA proteins in the drought response of Collembola.

Humidity has a large impact on the distribution and abundance of terrestrial invertebrates, but the molecular mechanisms governing drought resistance are not fully understood. Some attention has been given to the role of the heat shock response as a component of desiccation tolerance, but recent focus has been on the chaperone-like LEA (late embryogenesis abundant) proteins in anhydrobiotic animals. This study investigates the expression of putative LEA proteins as well as the heat shock protein Hsp70 during drought stress in soil and surface dwelling species of Collembola (springtails). In silico analysis of four EST candidates from two species of Collembola showed the presence of a Group 3 LEA protein in Megaphorura arctica. In common with other Group 3 LEA proteins, the new sequence is predicted to be 100% natively unfolded, with a strong degree of lysine and alanine periodicity and with a negative average hydrophobicity of -1.273. The sequence clusters with members of the Group 3 LEA in plants. Furthermore, cross-species Western blotting showed drought-induced expression of putative LEA proteins in six species of Collembola. In the surface dwelling species, Orchesella cincta, degree of dehydration and length of exposure correlated with level of putative LEA protein. Hsp70 was also found to increase in individuals of O. cincta and Folsomia candida that had been exposed to drought conditions for 6 days. These results show the presence of a LEA protein-coding region in Collembola, but also indicate that several proteins are involved in response to dehydration stress, including Hsp70.

Functional divergence of former alleles in an ancient asexual invertebrate.

Theory suggests it should be difficult for asexual organisms to adapt to a changing environment because genetic diversity can only arise from mutations accumulating within direct antecedents and not through sexual exchange. In an asexual microinvertebrate, the bdelloid rotifer, we have observed a mechanism by which such organisms could acquire the diversity needed for adaptation. Gene copies most likely representing former alleles have diverged in function so that the proteins they encode play complementary roles in survival of dry conditions. One protein prevents desiccation-sensitive enzymes from aggregating during drying, whereas its counterpart does not have this activity, but is able to associate with phospholipid bilayers and is potentially involved in maintenance of membrane integrity. The functional divergence of former alleles observed here suggests that adoption of asexual reproduction could itself be an evolutionary mechanism for the generation of diversity.

Dehydration-induced expression of LEA proteins in an anhydrobiotic chironomid.

Late embryogenesis abundant (LEA) proteins are associated with desiccation tolerance in anhydrobiotic organisms. The larvae of an African chironomid, Polypedilum vanderplanki, are able to withstand almost complete desiccation during which they enter a state of suspended animation. Here, we developed an EST database from desiccating larvae and isolated three cDNAs encoding proteins (PvLEA1, PvLEA2, and PvLEA3) with highly significant matches to Group 3 LEA proteins. Both mRNA and protein levels of all three examples were increased by dehydration stress imposed by either desiccation or hypersalinity, and one protein, PvLEA2, is likely to be post-translationally processed into smaller molecules. This first description of LEA protein genes in arthropods suggests that this protein family is widespread throughout invertebrate phyla, and that animals, plants, and microorganisms possess similar mechanisms for combating dehydration stress.