Peptides as signals - Intercellular Communication in Plants - page 40

resistance response. Such a response would cause the papillae to secret components
that could be detrimental to pollen.

2.3 Proteolytic processing of prohormones

Insulin was the first hormone found to be derived from a precursor (Steiner & Oyer,
1966). Later discoveries revealed that multiple peptide hormones can be produced
from a common precursor (Mains et al. , 1977). Only 15 years ago the first prohor-
mone processing enzyme (Kex2) was identified in yeast (Fuller et al. , 1989). It is
now known that peptide signals are generally synthesized as larger inactive precur-
sors that undergo limited proteolysis to yield active peptide signals. Such proteolytic
processing is one of many mechanisms that regulate activity of peptide hormones.
Proteolytic processing of prohormones is catalyzed by five endopeptidases (com-
monly termed proteases): serine, cysteine, aspartic, metallo-, and threonine proteases
(Barrett, 1998). Exopeptidases may also be involved in the process. For instance,
consider the case of angiotensin II, the most important animal hormone in regulating
blood pressure, and water and salt balance. Angiostensin II is derived from cleavage
of its 55-kDa precursor by rennin (an aspartic protease) to generate the decapep-
tide angiotensin I, which is then processed by a carboxypeptidase to remove two
carboxyterminal amino acids. Renin is the rate-limiting enzyme in the proteolytic
cascade (Morris, 2003).
Most of the known peptide signals in plants are derived from large precursors,
including systemins, RALFs, and PSKs. All of those peptide signals were initially
identified using biochemical approaches based on their biological activities. It re-
mains to be determined if CLV3, SCR, or ENOD40 also undergoes posttranslational
proteolytic processing to generate a biologically active signal molecule.
To date, no protease has been identified in the maturation of the plant peptide
signals. Proteolytic processing of prosystemin is thought to take place as a result
of wounding, which brings prosystemin together with protease(s) from another
cellular compartment. An aspartic protease is induced during the systemic wound
response and could be involved in processing of prosystemin; however, no direct
evidence exists to support that notion (Schaller & Ryan, 1996). Many prohormones
in animals contain a pair of basic amino acids prior to and/or after the sequences
of the mature hormones. Such dibasic residues are the preferred cleavage sites for
prohormone-processing serine proteases. The Arabidopsis PSK precursors contain
such dibasic residues near both sides of the PSK sequences (Yang et al. , 2001).
Therefore, the PSK precursors may be cleaved at the dibasic sites by unknown
serine proteases and further trimmed by exopeptidases to form mature PSKs. The
Arabidopsis genome encodes over an estimated 550 putative proteases (Beers et al. ,
2004). Future studies will likely lead to identification of many of them as prohormone
maturation enzymes.
Again-of-function genetic screen aimed at isolating components of the BRI1-
mediated signaling pathway has resulted in the identification of an extracellular