PROSITE documentation PDOC50842

Expansin family profiles

Expansins are secreted proteins of 25 to 27 Kd that were isolated first from young cucumber seedling and subsequently from other plant tissues. Expression of expansin genes correlates with growth of cells. Increase in expansin content also occurs during fruit ripening. Expansins act on the cell wall to promote its extensibility. The model for its mechanism of action postulates that expansins break non-covalent bonds between cell-wall polysaccharides, thereby permitting pressure dependent expansion of the cell [1,2].

Group-I pollen allergens of grasses have limited but significant sequence homology to expansin. These proteins are the main causative agent of hay fever and seasonal asthma induced by grass pollen. Extracts containing group-I allergens are also active in loosening cell-walls. Group-I pollen allergens and related proteins in vegetative tissues have been classified as β-expansins, whereas the earlier discovered expansins are now referred to as α-expansins [3].

Expansin-like proteins are also found in some fungi. In Trichoderma reesei an expansin-like protein (Cel12A) acts as a glycoside hydrolase on xyloglucan and 1-4 β-glucan. These hydrolytic actions differ from the action by expansins, which induce wall extension by a non-hydrolytic mechanism [4].

Expansins consist of two domains closely packed and aligned so as to form a long, shallow groove with potential to bind a glycan backbone of ~10 sugar residues (see <PDB:2HCZ>). The N-terminal cysteine-rich domain has distant sequence similarity to family-45 endoglucanases (EG45-like domain). It has noteworthy, but incomplete, conservation of the catalytic site identified in EG45 enzymes. The ~120-residue EG45-like domain has three disulfide bonds and the six participating cysteines are highly conserved. Its fold is dominated by a six-stranded β-barrel flanked by short loops and α-helices. The ~90-residue C-terminal domain may function as a cellulose-binding domain (CBD). It is composed of eight β-strands assembled into two antiparallel β-sheets. The two β-sheets are at slight angles to each other and form a β-sandwich similar to the Ig fold [5].

Two profiles were developed for this family, the first one picks up the EG45-like domain while the second profile detects the cellulose-binding domain.