EXPANSINS

Biochemical dissection of the ''acid-growth'' process of plant cell wa lls led to the isolation of a new class of wall loosening proteins, ca lled expansins. These proteins affect the theology of growing walls by permitting the microfibril-matrix network to slide, thereby enabling the wall to expand. Molecular sequence analysis suggests that expansin s might have a cryptic glycosyl transferase activity, but biochemical results suggest that expansins disrupt noncovalent bonding between mic rofibrils and the matrix. Recent discoveries of a new expansin family and gene expression in fruit, meristems and cotton fibers have enlarge d our view of the developmental functions of this group of wall loosen ing proteins.