Molecular characterization of a xyloglucan-specific endo-(1-->4)-beta-D-glucanase (xyloglucan endo-transglycosylase) from nasturtium seeds.

A novel xyloglucan-specific endo-(1-->4)-beta-D-glucanase, involved in the post-germinative mobilization of xyloglucan storage reserves, has previously been isolated from nasturtium (Tropaeolum majus L.) seed. Its mode of action has been shown, in vitro, to be one of transglycosylation except at low substrate (glycosylacceptor) concentrations when hydrolysis predominates. Here it is shown that this nasturtium seed xyloglucan endo-transglycosylase is encoded by a single gene which is transcribed and processed to a 1.5 kb mRNA. The isolation and DNA sequence analysis of a cDNA copy of the nasturtium xyloglucan endo-transglycosylase transcript is described. The cDNA encodes a 33.5 kDa precursor polypeptide which is subsequently processed to a 31 kDa mature protein. The precursor incorporates an N-terminal signal sequence which probably contains information relevant to the targeting of the enzyme to the cell wall. The computer-predicted isoelectric point (5.14) and low (approximately 0%) alpha-helix content of the deduced mature protein are in excellent agreement with the experimental data obtained using the purified enzyme. The deduced protein sequence lacks homology with known plant endo-(1-->4)-beta-D-glucanases, consistent with the unique properties of the enzyme. Database searches have revealed that a Brassica protein (meri-5) of previously unknown function, but abundantly expressed in expanding tissue, shares structural identity with the nasturtium xyloglucan endo-transglycosylase. The expression of a xyloglucan endo-transglycosylase in expanding tissue would be consistent with the contention that enzymes of this type are involved in cell wall loosening.