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or signal transduction and
are
active against the fungus
Blirmeria graminis
which
causes powdery mildew on barley. Recognized families of PR proteins are listed in
Table I.
2.2.1. Biochemical a n d structural characteristics of PR-proteins
PR-proteins are distinguished by specific biochemical properties. They are
low-molecular weight proteins (6-43
kDa),
extractable and stahle at low pH
(c
3).
thermostable and highly resistant to proteases (Van Loon, 1999). The structure of a
PR-1 family member (tomato PRl-b) was solved by nuclear magnetic resonance
(NMR) and was found to represent a unique molecular architecture. The protein
contains four a-helices and four P-strands arranged antiparallel between helices. The
tight packing of the a-helices on both sides of the central P-sheet (a-@-a sandwich
structure) results in a compact, bipartite molecular core, which is stabilized by
hydrophobic interactions and multiple hydrogen bonds (Fernsndez
er ol.,
1997). This
compact structure probably determines the high stability of PR-proteins and their
insensitivity to proteases.
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