Poster: Cell Walls

Abs # 1275: Crystal structure of Zea m 1, a group-1 grass pollen allergen and b-expansin from maize

Presenter:	Yennawar, Neela J
Authors	Yennawar, Neela J (A)   Li, Lian Chao  (A)   Yennawar, Hemant  (A)   Cosgrove, Daniel J (B)
Affiliations:	(A): Department of Biochemistry and Molecular Biology, Penn State University (B): Department of Biology, Penn State University

Zea m 1 is a member of the b-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 determined its structure by X-ray crystallography using molecular replacement with the Phl p 1 model. 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). Compared with EGV which has a deep 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. 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. 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.