This is a Jmol presentation on the plant antifungal protein, osmotin. Presentation by Marielle Nicole A. Cajita, Department of Chemistry, University of Massachusetts. December 6, 2004.

"You need proteins to make DNA, but you also need DNA to make proteins. Which came first? It is the classic chicken and egg problem."
-Catholics and the Debate Over Evolution

INTRODUCTION
is a 24 kDa protein belonging to the PR-5 protein family whose members are homologous to the sweet-tasting protein thaumatin. Osmotin and other PR-5 proteins were shown to have antifungal activity in vitro against a broad range of fungi, including several plant pathogens.

THE STRUCTURE
Osmotin is composed of three . The consists of 11-strand, flattened beta sandwich that forms the compact core of the molecule. This beta sandwich has an .

The consists of alpha helices extending from the first domain. This domain is stablized by .

The consists of a small loop with .

, which drives protein folding, are shown in purple. The green region represents the hydrophilic groups.

THE FUNCTION
Analysis of the electrostatic properties reveals an formed by Glu84, Asp97, Asp102, and Asp185. It can be assumed that the acidic cleft of osmotin may be involved in interaction with its receptor in the plasma membrane of fungi.

Although all of the known antifungal PR-5 proteins are similar in their overall structures and in the acidic cleft regions, they show different activities and specificities against their target cells. Based on the assumption that the acidic cleft is important for its antifungal activity and is relevant to receptor binding, differences of topology and surface electrostatic potential around the cleft are considered to determine the specificity of PR-5 proteins to their target cells.

ABSTRACT
To understand the structural basis of antifungal activity of osmotin, its crystal structure was determined at 2.3 A resolution and compared to the structure of other antifungal proteins and thaumatin. Osmotin was purified from salt-adapted tobacco cell suspension cultures (Nicotiana tabacum L. var. Wisconsin 38). The fungal growth inhibition by osmotin is correlated with plasma membrane permeabilization and dissipation of the plasma membrane potential, suggesting a physical interaction between PR-5 proteins and the plasma membrane of sensitive fungi. However, the precise mechanism by which osmotin interacts with specific fungal pathogens and mediates plasma membrane permeability has not been clearly elucidated.