Isolation and characterization of a proline-rich cell wall protein from soybean seedlings

We have purified a cell wall protein from extracts of soybean (Glycine max) that was previously shown to be immunologically related to p33, a wound-induced carrot cell wall protein (Tierney ML, Wiechert J, Pluymers D [1988] Mol Gen Genet 211: 393-399). Amino acid composition analysis reveals that the protein is 36% proline and hydroxyproline and its amino-terminus was determined to be AsnTyrGluAsnProHypValTyrLysProHypThrGluLysProHypValTyr. In vivo [(3)H]proline labeling of soybean hypocotyl tissues and Western blot analysis indicate that the apical hook, a region characterized by active growth, is a predominant site for synthesis of this protein and that wounding does not significantly affect its synthesis or accumulation. Antibodies raised against the hook protein also cross-reacted with a higher molecular weight protein in cell wall extracts of wounded hook tissue. These data indicate that there are at least two related, proline-rich cell wall proteins in the hook region of soybean seedlings, one that appears to be developmentally regulated and another that is wound-inducible. RNA encoding the developmentally regulated seedling cell wall protein was detected in both the hook and elongating region of the hypocotyl. The presence of this RNA in the elongating region in the absence of detectable levels of the protein indicates either that the expression of this cell wall protein is subject to posttranscriptional control or that the rate of insolubilization of this protein increases in more mature regions of the hypocotyl.