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Abstract. Recent research on fold-switching metamorphic proteins has revealed some notable exceptions to Anfinsen's hypothesis of protein folding. We have previously described how a single point mutation can enable a well-folded protein domain, one of the two PAS (Per-ARNT-Sim) domains of the human ARNT (aryl hydrocarbon receptor nuclear translocator) protein, to interconvert between two conformers related by a slip of an internal β strand. Using this protein as a test case, we advance the concept of a “fragile fold”, a protein fold that can reversibly rearrange into another fold that differs by a substantial number of hydrogen bonds, entailing reorganization of single secondary structure elements to more drastic changes seen in metamorphic proteins. Here we use a battery of biophysical tests to examine several factors affecting the equilibrium between the two conformations of the switching ARNT PAS-B Y456T protein. Of note is that we find that factors which impact the HI loop preceding the shifted Iβ strand affect both the equilibrium levels of the two conformers and the denatured state which links them in the interconversion process. Finally, we describe small molecules that selectively bind to and stabilize the wild-type conformation of ARNT PAS-B. These studies form a toolkit for studying fragile protein folds and could enable ways to modulate the biological functions of such fragile folds, both in natural and engineered proteins. 

Many restoration projects' success is not evaluated, despite available conventional ecological assessment methods. There is a need for more flexible, affordable, and efficient methods for evaluation, particularly those that take advantage of new remote sensing and geospatial technologies. This study explores the use of illustrative small unmanned aerial system (sUAS) products, made using a simple structure‐from‐motion photogrammetry workflow, coupled with a visual assessment protocol as a remote evaluation and ecological condition archive approach. Three streams were assessed in the field (“surface assessments”) using the Stream Visual Assessment Protocol Version 2 (SVAP2) and later illustrated in sUAS products. A survey of 10 stream experts was conducted to (1) assess the general utility of the sUAS products (high‐resolution video, orthomosaics, and 3D models), and (2) test whether the experts could interpret the products and apply the 16 SVAP2 elements remotely. The channel condition, bank condition, riparian area quantity, and canopy cover elements were deemed appropriate for remote assessment, while the riparian area quality, water appearance, fish habitat complexity, and aquatic invertebrate complexity elements were deemed appropriate for remote assessment but with some potential limitations due to the quality of the products and varying site conditions. In general, the survey participants agreed that the illustrative products would be useful in stream ecological assessment and restoration evaluation. Although not a replacement for more quantitative surface assessments when required, this remote visual approach is suitable when more general monitoring is satisfactory.