ISOLATION AND CHARACTERIZATION OF 3 SOYBEAN EXTENSIN CDNAS

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 extensin SbHRGP-1 containing the 16-amino acid repeat Ser-Pro(4)-Ser-Pro-Ser-Pro(4)-Tyr-Val-Tyr-Lys, with Val occasionally replaced by Ile or Tyr. The second transcript encoded the SbHRGP-2 protein containing the 16-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 variant 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, respectively. 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.