Both families of expansins have been identified in a wide range of land plants, from angiosperms and gymnosperms to ferns and mosses.
Some animals, such as Globodera rostochiensis, a plant-parasitic nematode, can produce a functional expansin which uses it to loosen cell walls when invading its host plant.
Expansins are also expressed in ripening fruit where they function in fruit softening,[16] and in grass pollen,[7] where they loosen stigmatic cell walls and aid pollen tube penetration of the stigmain germinating seeds for cell wall disassembly,[17] in floral organs for their patterning, in developing nitrogen-fixing nodules in legumes, in abscessing leaves, in parasitic plants, and in ‘resurrection’ plants during their rehydration.
Generally speaking, α- and β-expansins and expansin-like are composed of approximately 300 amino acids,[9] with a MW of ~25–28 kDa for the mature protein.
The peptidic sequence of an expansin consists, in particular, of: a signal peptide of around 20–30 amino acids at the N-terminal end, the putative catalytic domain, a His-Phe-Asp (HFD) motif in central region (except EXL), and the C-terminal putative cellulose-binding domain with conserved Trp (tryptophan) residues.
[19] Within the cell wall, this expansion of surface area involves slippage or movement of cellulose microfibrils, which normally is coupled to simultaneous uptake of water.
Moreover, B. subtilis mutants lacking BsEXLX1 were defective in colonizing plant roots, suggesting that this protein facilitates plant-bacterium interactions.
[21] The expansin domain II, causative of the allergenic effects, could be related to the competition between pollens for access to ovules.