PLANT PHYSIOLOGY , Vol 107, Issue 4 1323-1332, Copyright © 1995 by American Society of Plant Biologists
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BIOCHEMISTRY AND ENZYMOLOGY |
Maize Ribosome-Inactivating Protein (b-32) (Homologs in Related Species, Effects on Maize Ribosomes, and Modulation of Activity by Pro-Peptide Deletions)
T. D. Hey, M. Hartley and T. A. Walsh
Biotechnology Department, DowElanco, P.O. Box 68955, 9330 Zionsville Road, Indianapolis, Indiana 46268-1053 (T.D.H., T.A.W.)
The ribosome-inactivating protein (RIP) from maize (Zea mays L.) is unusual
in that it is produced in the endosperm as an inactive pro-form, also known
as b-32, which can be converted by limited proteolysis to a two-chain
active form, [alpha][beta] RIP. Immunological analysis of seed extracts
from a variety of species related to maize showed that pro/[alpha][beta]
forms of RIP are not unique to maize but are also found in other members of
the Panicoideae, including Tripsacum and sorghum. Ribosomes isolated from
maize were quite resistant to both purified pro- and [alpha][beta] maize
RIPs, whereas they were highly susceptible to the RIP from pokeweed. This
suggests that the production of an inactive pro-RIP is not a mechanism to
protect the plant's own ribosomes from deleterious action of the
[alpha][beta] RIP. RIP derivatives with various pro-segments removed were
expressed at high levels in Escherichia coli. Measurement of their activity
before and after treatment with subtilisin Carlsberg clearly identified the
25-amino acid intradomain insertion, rather than the N- or C-terminal
extensions, as the major element responsible for suppression of enzymatic
activity. A RIP with all three processed regions deleted had activity close
to that of the native [alpha][beta] form.
