Polypeptide Hormones

Abstract

Polypeptide hormones (extracellular signals) have been recognized for many years as important regulatory molecules in animals and yeast. Beginning with the discovery of insulin in 1922 (Banting and Best, 1922), polypeptides have been established as signals that regulate a broad spectrum of physiological processes. Until 1991, however, plants were not known to utilize polypeptides as regulatory molecules; instead they were thought to use only small organic phytohormones, including auxins, cytokinins, gibberellins, ethylene, abscisic acid (Kende and Zeevaart, 1997), and more recently, brassinolides (Mandava, 1988). The discoveries of polypeptide signals for plant defense, growth, and development over the past decade have heralded the beginning of a new and growing field of polypeptide signaling in plants. Here, we review the currently known polypeptides from plant origins that act as signaling molecules. Where known, the receptor proteins with which the polypeptides interact are discussed, as well as their functional roles in polypeptide/receptor–mediated signaling cascades. This article is not meant to be a comprehensive review of the underlying biology of systems in which polypeptide signaling is found, and our discussion of the relevant research areas emphasizes the biochemical and structural features of polypeptide signals and their receptors. The systemin polypeptide was discovered during a search for the systemic wound signal that regulates the expression of defensive genes in tomato leaves in response to insect attacks or other severe mechanical wounding (Pearce et al., 1991). Several reviews concerning tomato systemin have been written in recent years (Schaller and Ryan, 1995; Ryan, 1998, 2000; Ryan and Pearce, 1998; Schaller, 1999), and the structural and functional data relevant to polypeptide signaling, along with more recent developments, are summarized below. Tomato and potato plants respond to insect attacks by releasing a signal or signals from the wound site that cause the synthesis and accumulation of proteinase inhibitors in both wounded leaves and distal, unwounded leaves (Green and Ryan, 1972). A search for the proteinase inhibitor-inducing factor over the next 20 years led to the isolation of a polypeptide signaling molecule (Pearce et al., 1991; Ryan, 1998). The polypeptide is composed of 18 amino acids with the sequence +AVQSKPPSKRDPPKMQTD−, and because of its association with systemic signaling, it is called systemin. Systemin induces the synthesis of proteinase inhibitors in leaves of young excised tomato plants when supplied at low nanomolar levels through their cut petioles (Pearce et al., 1991). The potency is in the same range as that found for the activity of various polypeptide hormones in animals. A synthetic 14C-labeled systemin was mobile when applied to wound sites, and its movement in the phloem correlates with the movement of the systemic signal that is produced in response to wounding, that is, ∼3 cm/hr (Nelson et al., 1983; Pearce et al., 1991; Narvaez-Vasquez et al., 1995). Prosystemin is found in the vascular bundles of tomato petioles (Jacinto et al., 1997). The mechanism for the movement of systemin in the phloem is not understood, but for systemin to have long-range signaling effects, an amplification of the signal most likely occurs. The presence of prosystemin in phloem parenchyma cells (J. Narvaez-Vasquez and C.A. Ryan, unpublished data) suggests that an interplay between the phloem and its surrounding cells may contribute to the signaling mechanism. Systemin contains several charged amino acids as well as two proline doublets, at positions 6 and 7 and at 12 and 13. Only weak structural features are present in aqueous solutions of systemin, analyzed using nuclear magnetic resonance spectroscopy (Russell et al., 1992), but circular dichroism spectra reveals a poly(l-proline) II, 31 helix secondary structure in the central region of the polypeptide (Toumadje and Johnson, 1995). The polyproline helix is known to provide a kink in proteins (Ferris et al., 2001) and may provide a structure in systemin that is important to its recognition by the systemin receptor (discussed below). Sequential alanine substitution at each of the 18 amino acids of systemin has variable effects on activity. A Thr to Ala substitution at position 17 eliminates activity (Pearce et al., 1993a) and creates a powerful antagonist of native systemin. Removal of the C-terminal Asp of systemin abolishes activity, and this analog is also an antagonist of systemin. Substitution of Pro12 and Pro13 residues with Ala severely reduces the activities of the analogs compared with that of native systemin. The Ala substitution experiments, together with the deletion experiments, indicated that the 14 amino acids at the N terminus are likely involved in receptor binding, whereas the MQTD sequence at the C terminus has a direct role in signaling (Pearce et al., 1993a). This conclusion is supported by experimen