Expansins are ubiquitous plant cell wall glycoproteins that catalyze cell wall loosening during cell growth and other developmental processes [1]. They are typically 250-275 amino acids long. Phylogenetic analysis revealed the existence of four subfamilies designated α-expansins, β-expansins, expansin-like A, and expansin-like B. Sequence identity between members of different subfamilies is only 20-40%. Expansin structures consist of two domains preceded by a poorly conserved unstructured glycosylated N-terminal stretch. The N-terminal domain (Pfam:PF03330) folds into a six-stranded β-barrel stabilized by three disulfide bonds. It is also found in some bacterial and eukaryotic lipoproteins and belongs to a large superfamily characterized by a six-stranded double-psi beta barrel fold [2]. The C-terminal domain (Pfam:PF01357), adopts an immunoglobulin-like two-layered β-sandwich structure.
β-Expansins from grass pollen are a family of major allergens designated group 1 grass pollen allergens. In addition, there is a family of expansin-related minor grass pollen allergens (group 2/3) that lack the N-terminal expansin domain. Weak IgE cross-reactivity between allergens from the groups 1 and 2/3 was observed [3]. No expansins outside the grass family have been shown to be allergenic.
Family-defining Pfam domains (at least one of these domains is present in each family member):
| Pfam domain | Pfam clan | ||
| PF01357 | Pollen allergen | - | |
| PF03330 | Lytic transglycolase | CL0199 | Double Psi beta barrel glucanase |
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