Identification of calreticulin-like protein as one of the phosphoproteins modulated in response to oligogalacturonides in tobacco cells

Oligogalacturonide-induced modifications of protein phosphorylation in cells of Nicotiana tabacum L. were investigated by in-vitro phosphorylation of plasma-membrane-enriched fractions and electrophoretic analysis on two-dimensional gels. About 100 polypeptides were resolved; among these 40 phosphoproteins were detected and their 33P-labelling quantified. Most of the phosphorylations were inhibited by staurosporine and several proteins were hyperphosphorylated in the presence of okadaic acid, indicating the presence of protein phosphatase(s) in addition to staurosporine-susceptible protein kinase(s) in the plasma-membrane-enriched fraction. In the presence of oligogalacturonides, phosphorylation of seven acidic polypeptides ranging from 15 to 65 kDa was strongly enhanced. A twofold enhancement of the phosphorylation of a 24-kDa protein and a two- to threefold decrease in the phosphorylation of acidic proteins of Mrs 62, 65, 80 and 84 was also observed in response to oligogalacturonides. One of the oligogalacturonide-modulated phosphoproteins was identified as calreticulin by direct nucleotide sequencing after preparative two-dimensional electrophoresis and comparison with protein database sequences. Decreased phosphorylation of calreticulin was also observed in vivo, shortly after addition of oligogalacturonides to tobacco cells, confirming the biological relevance of the modification. Although the presence of calreticulin, an abundent reticuloplasmin with high calcium-binding capacity, has been reported in both mammalian and plant cells, its function is as yet largely unknown. Modulation of the phosphorylation of a plant calreticulin-like protein by oligogalacturonides is shown here, suggesting a role in the early transduction steps of this signal.