Expression and processing of a hormonally regulated beta-expansin from soybean

Expansin proteins are essential components of acid-induced cell wall loosen ing in plants. beta -Expansins, which constitute a subfamily of related exp ansin proteins, include the group I grass pollen allergens. To provide a be tter description of beta -expansin expression, we have characterized a cyto kinin-inducible beta -expansin from soybean (Glycine I,max cv Mandarin) cal led Cim1. Our results demonstrate that the hormones cytokinin and auxin act synergistically to induce the accumulation and proteolytic processing of C im1. Carboxyl terminal truncation of a 35-kD form of Cim1 is predicted to r emove the putative cellulose binding domain from the amino terminal cystein e-rich domain, resulting in a 20-kD form of the protein. Furthermore, the i dentical amino termini of the 35- and 20-kD forms of Cim1 correspond to a p osition 11 amino acids downstream of the predicted signal sequence cleavage site, suggesting proteolysis of a short amino terminal propeptide after re moval:of the signal peptide. This propeptide fragment contains a consensus site for N-glycosylation and our data suggest that it is glycosylated by a tunicamycin-sensitive mechanism in cultured soybean cells. The onset of Cim 1 expression correlates with increased growth of soybean cultures. Ultimate ly, Cim1 is rapidly and specifically proteolyzed as soybean cultures reach stationary phase. These findings are consistent with the hypothesis that be ta -expansin proteins are extensively modified by post-translational N-glyc osylation and proteolysis.