Transformer proteins

Proteins are generally believed to adopt a unique fold, defined by their amino acid sequence, under specific environmental conditions. These unique structures, in turn, endow proteins with one specific function. However, not all proteins obey the “1 amino acid sequence → 1 fold → 1 function” scheme. Moonlighting proteins that adopt one distinct threedimensional structure but can accomplish two or more alternative functions, such as NusE/ S10, and proteins that assume two slightly different folds in identical environments have been described. Examples of the latter are the Aquifex aeolicus ribosomal protein L20 and the chemokine lymphotactin (Ltn). In either protein, the difference between the two conformational states is modest: in L20, a short nine-residue unstructured region alternatively folds into an α-helix, and in Ltn an N-terminal loop changes to a β-strand, while the C-terminal α-helix becomes unstructured. Differences in small parts of protein structures even in identical solution are, however, not uncommon. In fact, all molecules are subject to dynamic changes, but those have to be distinguished from genuine structural changes, a term which should be restricted to cases where the alternative states are different to a large extent; that is, they either have different stable standard secondary structures or different tertiary topologies. However, even the minor unstructured-structured transitions, as observed for L20 and Ltn, gave rise to the term metamorphic proteins. Gross structural changes were observed for prion proteins. They can adopt two structures; one state is soluble and functional, whereas another is insoluble and pathogenic. Yet, the two states do not Transformer proteins