Plant Molecular Biology 38: 1243–1246, 1998.
© 1998 Kluwer Academic Publishers. Printed in the Netherlands.
Sequential and structural homology between intracellular
pathogenesis-related proteins and a group of latex proteins
, Brian Boyle and Normand Brisson
Department of Biochemistry, Universit´e de Montr´eal, Montr´eal, Canada H3C 3J7 (
author for correspondence)
Received 18 March 1998; accepted 1 June 1998
Key words: Bet v 1, homology, latex proteins, pathogenesis-related proteins, PR-10, sequence analysis
The intracellular pathogenesis-related proteins have been identiﬁed in a broad range of ﬂowering plants. Some
display quite different patterns of expression, in many cases unrelated to the pathogenic response. Nevertheless,
these proteins are all very similar and in most cases share more than 35% sequence identity. In this report we
investigate the signiﬁcance of a rather weak similarity between the intracellular pathogenesis-related (IPR or PR-
10) proteins and a group of proteins identiﬁed in the latex of opium poppy and in Arabidopsis, among others. A
sequence analysis held together with the recently published three-dimensional structure of Bet v 1, an IPR protein
from birch pollen, strongly suggests sequential and structural homology between the two protein families.
The intracellular pathogenesis-related (IPR or PR-10
[13, 12]) proteins are a family of small homologous,
primarily acidic proteins which have been identiﬁed in
a variety of angiosperms, monocots as well as dicots.
Their biological function is basically unknown, al-
though expression patterns for many of the molecules
have been thoroughly studied and a few members
have been reported to possess RNase activity [1, 5].
Usually, these proteins accumulate in connection with
fungalandbacterialinfection, but severalareregulated
by other stress factors, such as drought, wounding,
and darkness, or they are developmentally regulated
in certain tissues. A number of the IPR proteins are
of immunological interest as they possess strong al-
lergenic properties. These include the major allergens
from birch and hornbeam pollen, apple, carrot, and
A weak sequence similarity between these proteins
and a family of proteins ﬁrst identiﬁed in the latex
of opium poppy (Papaver somniferum)ledusto
undertake a thorough examination of their mutual re-
lationship. These latter proteins have been designated
major latex proteins, or MLPs , and have later been
found in arabidopsis, bell pepper, melon, strawberry,
and tobacco. Also in this case the function of the pro-
teins in the plant is unknown, but a few show patterns
of expression similar to some of the IPR proteins.
Thus, thebell pepperproteins areknown to be induced
by wounding as are IPR proteins in potato ,soy-
bean  and asparagus . Furthermore, all of the
known MLPs, like the IPR proteins, have a theoretical
pI in the acidic range (4.75–6.65). Curiously, no plant
is so far known to contain both MLP and IPR proteins.
No IPR protein has, for example, ever been identi-
ﬁed in the extensively studied tobacco plant, although
several are present in potato, which is a close relative.
The sequence similarity between MLP and IPR
proteins has been noticed earlier , but the extent
of identity is so low, less than 25%, that it can not
alone be used to establish whether the two families are
homologous. There is, however, compelling evidence
that this is in fact the case when the sequence align-
ment is analyzed in detail and compared to the known
three-dimensionalstructureof one of the IPR proteins,
the birch pollen allergen Bet v 1  (PDB entry 1bv1).
Figure 1 shows the alignment of 12 IPR- and
6 MLP sequences (for clarity, only a representative
subset of the known sequences has been included).
The alignment, which is based on 94 IPR and 13
MLP sequences, was initially performed with the pro-