NMR of plant proteins

This article focuses on NMR as a tool to determine the structures, folding behavior, dynamics and interactions of plant proteins. At the outset, we should say that plant proteins are of course not different in their fundamental constitution and properties from the proteins of bacteria or animals, and the technologies used to characterize proteins are generally the same whatever their source. The functions of plant proteins vary widely, from defense to photosynthetic activity and nutrient storage functions, as well as in signaling and hormonal roles and we will cover the breadth of these diverse functions throughout this article with illustrative examples. Functions such as photosynthesis or amino acid nutrient sequestration in seed storage proteins are of course unique to plants, but many other functions are shared with proteins from other kingdoms of life.

Plant proteins have been particularly amenable to NMR studies because their high abundance facilitated many early studies before recombinant production of animal proteins was widely available. There are several advantages to working with plant proteins, including the ability to grow large amounts of source tissue, a lack of some of the ethical issues associated with isolating animal proteins, and to some extent fewer safety concerns than can occur with the handling of microbial proteins. The argument could also be made that plant proteins can be regarded, in general, as having the advantage of being more stable than bacterial or animal proteins. Indeed one family of plant proteins that is a particular focus of our laboratory, the cyclotides, is especially stable, and it was NMR-derived structures that provided the first insights into the origin of this stability.

In general, quantitative NMR structural studies are typically limited to proteins of up to approximately 40 kDa in molecular weight and X-ray methods are faster and more efficient for larger proteins. There have been a large number of NMR and X-ray studies of plant proteins done over recent years and the distribution of structures solved between these two techniques is roughly similar between plant proteins and other sources of proteins, i.e., about 90% of structures have been determined by X-ray crystallography and about 10% have been determined by NMR. Since our focus is on NMR of plant proteins, we will not cover X-ray structures here, except to compare results from a few cases where NMR and X-ray studies have been made on the same protein.

Table 1 summarizes current data on plant proteins that have been structurally characterized by NMR and documented in the Protein Data Bank (PDB) and/or Biological Magnetic Resonance Bank (BMRB). The proteins range in size from approximately 10–300 amino acids and come from a wide range of plant sources, from herbs to shrubs to trees. The table provides a convenient summary of the current state-of-the-art in relation to structurally characterized proteins, and Sections 3–10 of this article describe illustrative examples drawn from this table. These examples are grouped into major categories of plant proteins, including defense proteins, electron transport proteins, photoreceptors, transcription factors, storage proteins, transfer proteins, proteins involved in metabolism, and plant proteins of human interest. Before discussing these examples, Section 2 gives a brief introduction to the other types of information (aside from structures) that can be derived from NMR experiments, again with illustrative examples being described in more detail in Sections 3–10.