Poster: Cell walls

Abs # 626: Crystal structure of Zea m 1, a group-1 grass pollen allergen and beta-expansin from maize.

Presenter:	Cosgrove, Daniel J , fsl@psu.edu
Authors	Yennawar, Neela J (A)   Li, Lian Chao  (A)   Yennawar, Hemant  (A)   Cosgrove, Daniel J  (A)
Affiliations:	(A): Penn State University
Web Site:	http://www.bio.psu.edu/expansins

Zea m 1 is a member of the beta-expansin subfamily known as group-1 grass pollen allergens. It has wall-loosening activity with a high specificity to grass cell walls. To elucidate its mechanism of wall loosening, we have crystallized native Zea m 1 and determined its structure by X-ray crystallography. Zea m 1 has two domains connected by a short linker. Domain 1 has substantial structural similarity to the catalytic domain of an endoglucanase (Humicola EGV) from family GH45. Residues His105, Thr25, Asp107 and Asn97 in Zea m 1 form a hydrogen-bonded network resembling the EGV active site (formed by His119, Thr6 and Asp121 in EGV). However, the second aspartic acid residue that is necessary for EGV enzymatic activity is missing in Zea m 1. These structural similarities suggest that domain 1 may have endoglucanase activity, but this is not supported by in-vitro assays. Also, EGV lacks the wall extension activity characteristic of expansins. Compared with EGV which has a deep glucan-binding cleft, the active site in Zea m 1 domain 1 is much more open, reducing steric hindrance for access to large polysaccharide structures, such as a cellulosic surface. Domain 2 of Zea m 1 is a immunoglobulin-like beta sandwich, essentially identical to that of Phl p 2, a group-2 grass pollen allergen. Domain 2 has aromatic and polar residues that form potential surfaces for polysaccharide binding. The structure of Zea m 1 leads to various hypotheses about its mechanism of cell wall loosening, which will be presented and evaluated.