Structure and function of plant cell wall proteins.

Plant cell walls are amazingly complex amalgams of carbohydrates, proteins, lignin, water, and incrusting substances such as cutin, suberin, and certain inorganic compounds that vary among plant species, cell types, and even neighboring cells. Developmental events and exposure to any of a number of abiotic and biotic stresses further increase this compositional and structural variation. Moreover, the dynamic nature and functions of plant cell walls in terms of growth and development, environmental sensing and signaling, plant defense, intercellular communication, and selective exchange interfaces are reflected in these variations. Much is currently known about the structure and metabolic regulation of the various cell wall components, but relatively little is known about their precise functions and intermolecular interactions. In this review, I will discuss the accumulated structural and regulatory data and the much more limited functional and intermolecular interaction information on five plant cell wall protein classes. These five protein classes, listed in Table 1, include the extensins, the glycine-rich proteins (GRPs), the proline-rich proteins (PRPs), the solanaceous lectins, and the arabinogalactan proteins (AGPs). These five proteins may be evolutionarily related to one another, most obviously because each of them, with the exception of the GRPs, contains hydroxyproline, and less obviously in the case of the GRPs because this class has nucleotide sequence similarity to the extensins. For completeness, I should mention that these are not the only cell wall proteins that are known. Others exist, such as cysteine-rich thionins, 28and 70-kD water-regulated proteins, a histidine-tryptophan-rich protein, and many cell wall enzymes such as peroxidases, phosphatases, invertases, a-mannosidases, pmannosidases, p1,3-glucanases, p1,4glucanases, polygalacturonase, pectin methylesterases, malate dehydrogenase, arabinosidases, a-galactosidases, pgalactosidases, pglucuronosidases, pxylosidases, proteases, and ascorbic acid oxidase (Varner and Lin, 1989). However, the above five classes generally represent the most abundant, and to date, the most well-studied and widely documented, plant cell wall proteins. Before describing these five wall protein classes, I should point out that research on these individual proteins has occurred in severa1 plant species, but relatively few examples exist where these cell wall proteins have been studied together in one plant, let alone in one particular plant organ or type of cell. Thus, data from one plant species are often extrapolated to represent the situation in other plant species. Although such extrapolations are usually valid, enough variations are now known that caution should be exercised in making or believing such claims.