We have characterized three different soybean (Glycine max) mRNAs that
encode apoproteins of extensins, a family of cell wall hydroxyproline
-rich glycoproteins (HRGPs). These transcripts encoded distinctive Tyr
-rich proteins containing characteristic Ser-Pro, sequences organized
in higher-order repetitive units. The first transcript encoded an exte
nsin SbHRGP-1 containing the 16-amino acid repeat Ser-Pro(4)-Ser-Pro-S
er-Pro(4)-Tyr-Val-Tyr-Lys, with Val occasionally replaced by Ile or Ty
r. The second transcript encoded the SbHRGP-2 protein containing the 1
6-amino acid repeat Ser-Pro(4)-Ser-Pro-Ser-Pro(4)-Tyr-Tyr-Tyr-Lys/His.
The third transcript encoded the SbHRGP-3 protein containing a varian
t of 9- or 10-amino acid canonical repeats: Ser-Pro(4)-Tyr-Lys-Tyr-Pro
, Ser-Pro(5)-Tyr-Lys-Tyr-Pro, and Ser-Pro(4)-Val-Tyr-Lys-Tyr-Lys, resp
ectively. The dramatic amino acid substitutions in the Tyr-rich blocks
(Tyr-X-Tyr-Lys) among these HRGPs indicate that each SbHRGP may have
a different function in cell wall architecture.