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Plant Physiol, March 2002, Vol. 128, pp. 822-832
Temperature-Induced Extended Helix/Random Coil Transitions in a
Group 1 Late Embryogenesis-Abundant Protein from
Soybean1
Jose L.
Soulages,
Kangmin
Kim,2
Christina
Walters, and
John C.
Cushman*
Department of Biochemistry and Molecular Biology, 355 Noble
Research Center, Oklahoma State University, Stillwater, Oklahoma
74078-0454 (J.L.S.); Department of Biochemistry, 311B Fleischmann
Agriculture, University of Nevada, Reno, Nevada 89557-0014 (K.K.,
J.C.C.); and National Seed Storage Laboratory, United States Department
of Agriculture-Agricultural Research Service, Fort Collins, Colorado
80523 (C.W.)
Group 1 late embryogenesis-abundant (LEA) proteins are a
subset of hydrophilins that are postulated to play important roles in
protecting plant macromolecules from damage during freezing, desiccation, or osmotic stress. To better understand the putative functional roles of group 1 LEA proteins, we analyzed the structure of
a group 1 LEA protein from soybean (Glycine max).
Differential scanning calorimetry of the purified, recombinant protein
demonstrated that the protein assumed a largely unstructured state in
solution. In the presence of trifluoroethanol (50% [w/v]),
the protein acquired a 30%  -helical content, indicating that the
polypeptide is highly restricted to adopt -helical structures. In
the presence of sodium dodecyl sulfate (1% [w/v]), 8% of the
polypeptide chain adopted an -helical structure. However, incubation
with phospholipids showed no effect on the protein structure.
Ultraviolet absorption and circular dichroism spectroscopy revealed
that the protein existed in equilibrium between two conformational
states. Ultraviolet absorption spectroscopy studies also showed that
the protein became more hydrated upon heating. Furthermore, circular
dichroism spectral measurements indicated that a minimum of 14% of
amino acid residues existed in a solvent-exposed, left-handed extended
helical or poly (L-proline)-type (PII) conformation at
20°C with the remainder of the protein being unstructured. The
content of PII-like structure increased as temperature was lowered. We
hypothesize that by favoring the adoption of PII structure, instead of
the formation of -helical or  -sheet structures, group 1 LEA
proteins retain a high content of surface area available for
interaction with the solvent. This feature could constitute the basis
of a potential role of LEA proteins in preventing freezing,
desiccation, or osmotic stress damage.
1
This work was supported by the United States
Department of Agriculture, National Research Initiative-Competitive
Grant Program (grant no. 99-35100-7004 to J.C.C.), by the National
Institutes of Health (grant no. GM 55622 to J.L.S.), and by the
Oklahoma and Nevada Agricultural Experiment Stations. The materials
described in this manuscript will be distributed publicly upon request
by contacting the corresponding author.
2
Present address: 193 E.R. Madigan Laboratory, 1201 W. Gregory Avenue, Urbana, IL 61801.
*
Corresponding author; e-mail jcushman{at}unr.edu; fax
775-784-1650.
© 2002 American Society of Plant Physiologists
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