J. Biol. Chem., Vol. 261, Issue 16, 7191-7195, Jun, 1986
Structural differences in the two major wheat germ agglutinin isolectins
CS Wright and S Olafsdottir
We have combined amino acid sequence data with x-ray diffraction results to
determine differences in structure of wheat germ agglutinin isolectin 1
(WGA1) relative to the known structure of wheat germ agglutinin isolectin 2
(WGA2). Electron density difference maps computed at 2.2 A resolution with
coefficients [2F(WGA1) - F(WGA2)] and [F(WGA1) - F(WGA2)] and based on
refined model phases of the WGA2 structure have revealed that the largest
differences in the two isolectin structures are localized in the B-domain
of the molecule. Amino acid sequence studies of tryptic and thermolytic
peptides of WGA1 confirm the strong homology between the two isolectins and
suggest variability at only four sequence positions. Three of these are
closely spaced in domain B. The two histidines in WGA2, His59 and His66,
are substituted by Gln and Tyr, respectively, and Pro56, by Thr in WGA1.
The fourth difference at position 93 in domain C was identified as a change
from Ser (WGA2) to Ala (WGA1). With these substitutions WGA1 exhibits a
slightly higher degree of internal homology than does WGA2. In addition, we
have carried out fluorescence studies on tryptic peptide T-3 to confirm the
presence of a second Trp residue in the wheat germ agglutinin molecule,
recently predicted at position 41 during the course of high resolution
crystal structure refinement of WGA2.
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