Expansins are relatively conserved proteins with the following features:

The signal peptide directs the nascent polypeptide into the ER/Golgi secretory pathway. This part of the protein (typically 22-25 amino acids) is removed as the protein enters the ER. 

The mature protein is ~25-27 kDa and consists of two domains, an amino-terminal domain of ~120 amino acid residues (green in structure below) with structural and sequence similarity to family-45 endoglucanases (EG45-like domain) and a carboxy-terminal domain of ~98 amino acid residues (cyan in structure below) that is hypothesized to function as a polysaccharide-binding domain (this is not experimentally established).

Below is a crystal structure model (PDB code 2HCZ) of a beta-expansin in which the two domains are colored green and cyan (Yennawar et a. 2006 PNAS 103:14664-14671). The two domains form a long, open planar surface with conserved polar residues that could hydrogen bond to a polysaccharide and four conserved aromatic residues (red below) that could bind the sugar rings by Van der Waals forces.

The image below shows a space-filling view of a beta-expansin (PDB code 2HCZ) with a polysaccharide (yellow/red stick figure)  aligned to the highly conserved planar surface (colored dark red; green=domain 1; cyan=domain 2). See Yennawar et al. 2006 PNAS 103:14664-14671.

This page was last updated on 03/22/06.

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