Grass group I allergens (beta-expansins) are novel, papain-related proteinases

Expansins are a family of proteins that catalyse long-term extension of iso lated plant cell walls due to an as yet unknown biochemical mechanism. They are divided into two groups, the alpha-expansins and beta-expansins, the l atter group consisting of grass group I allergens and their vegetative homo logs. These grass group I allergens, to which more than 95% of patients all ergic to grass pollen possess IgE antibodies, are highly immunologically cr ossreactive glycoproteins exclusively expressed in pollen of all grasses. A lignments of the amino-acid sequences of grass group I allergens derived fr om diverse grass species reveal up to 95% homology. It is therefore likely that these molecules share a similar biological function. The major grass g roup I allergen from timothy grass (Phleum pratense), Phl p 1, was chosen a s a model glycoprotein and expressed in the methylotrophic yeast Pichia pas toris to obtain a post-translationally modified and functionally active all ergen. The recombinant allergen exhibited proteolytic activity when assayed with various test systems and substrates, which was also subsequently demo nstrated with the natural protein, nPhl p 1. These observations are confirm ed by amino-acid alignments of Phl p 1 with three functionally important se quence motifs surrounding the active-site amino acids of the C1 (papain-lik e) family of cysteine proteinases. Moreover, the significantly homologous a lpha-expansins mostly share the functionally important C1 sequence motifs. This leads us to propose a C1 cysteine proteinase function for grass group I allergens, which may mediate plant cell wall growth and possibly contribu tes to the allergenicity of the molecule.