THE ARABIDOPSIS TCH4 XYLOGLUCAN ENDOTRANSGLYCOSYLASE - SUBSTRATE-SPECIFICITY, PH OPTIMUM, AND COLD TOLERANCE

Xyloglucan endotransglycosylases (XETs) modify a major component of th e plant cell wall and therefore may play critical roles in generating tissue properties and influencing morphogenesis. An XET-related gene f amily exists in Arabidopsis thaliana, the members of which show differ ential regulation of expression. TCH4 expression is rapidly regulated by mechanical stimuli, temperature shifts, light, and hormones. As a f irst step in determining whether Arabidopsis XET-related proteins have distinct properties, we produced recombinant TCH4 protein in bacteria and determined its enzymatic characteristics. TCH4 specifically trans glycosylates only xyloglucan. The enzyme prefers to transfer a portion of a donor polymer onto another xyloglucan polymer (acceptor); TCH4 w ill also utilize xyloglucan-derived oligosaccharides as accepters but discriminates between differentially fucosylated oligosaccharides. TCH 4 is most active at pH 6.0 to 6.5 and is surprisingly cold-tolerant wi th an optimum of 12 to 18 degrees C. TCH4 activity is enhanced by urea and bovine serum albumin, but not cations, reducing agents, or carbox ymethylcellulose. These studies indicate that TCH4 is specific for xyl oglucan, but that the molecular mass and the fucosyl content of the su bstrates influence enzymatic reaction rates. TCH4 is unlikely to play a role in acid-induced wall loosening but may function in cold acclima tion or cold-tolerant growth.