Small molecules that recognize protein surfaces are important tools for modifying protein interaction properties. Since the 1980s, several thousand studies concerning calixarenes and host-guest interactions have been published. Although there is growing interest in protein-calixarene interactions, only limited structural information has been available to date. We now report the crystal structure of a protein-calixarene complex. The water-soluble p-sulfonatocalix[4]arene is shown to bind the lysine-rich cytochrome c at three different sites. Binding curves obtained from NMR titrations reveal an interaction process that involves two or more binding sites. Together, the data indicate a dynamic complex in which the calixarene explores the surface of cytochrome c. In addition to providing valuable information on protein recognition, the data also indicate that the calixarene is a mediator of protein-protein interactions, with potential applications in generating assemblies and promoting crystallization.

Physical, chemical and biological stress factors, such as microbial infection, upregulate the transcription levels of a number of plant genes, coding for the so‐called pathogenesis‐related (PR) proteins. For PR proteins of class‐10 (PR‐10), the biological function remains unclear, despite two decades of scientific research. PR‐10 proteins have a wide distribution throughout the plant kingdom and the class members share size and secondary structure organization. Throughout the years, we and other groups have determined the structures of a number of PR‐10 proteins, both in the crystalline state by X‐ray diffraction and in solution by NMR spectroscopy. Despite the accumulating structural information, our understanding of PR‐10 function is still limited. PR‐10 proteins are rather small (~ 160 amino acids) with a fold consisting of three α helices and seven antiparallel β strands. These structural elements enclose a large hydrophobic cavity that is most probably the key to their functional relevance. Also, the outer surface of these proteins is of extreme interest, as epitopes from a PR‐10 subclass cause allergic reactions in humans.

No abstract

No abstract

Plant pathogenesis‐related (PR) proteins of class 10 are the only group among the 17 PR protein families that are intracellular and cytosolic. Sequence conservation and the wide distribution of PR‐10 proteins throughout the plant kingdom are an indication of an indispensable function in plants, but their true biological role remains obscure. Crystal and solution structures for several homologues have shown a similar overall fold with a vast internal cavity which, together with structural similarities to the steroidogenic acute regulatory protein‐related lipid transfer domain and cytokinin‐specific binding proteins, strongly indicate a ligand‐binding role for the PR‐10 proteins. This article describes the structure of a complex between a classic PR‐10 protein [Lupinus luteus (yellow lupine) PR‐10 protein of subclass 2, LlPR‐10.2B] and N,N′‐diphenylurea, a synthetic cytokinin. Synthetic cytokinins have been shown in various bioassays to exhibit activity similar to that of natural cytokinins. The present 1.95 Å resolution crystallographic model reveals four N,N′‐diphenylurea molecules in the hydrophobic cavity of the protein and a degree of conformational changes accompanying ligand binding. The structural adaptability of LlPR‐10.2B and its ability to bind different cytokinins suggest that this protein, and perhaps other PR‐10 proteins as well, can act as a reservoir of cytokinin molecules in the aqueous environment of a plant cell.