Identification of continuous, allergenic regions of the major shrimp allergen pen a 1 (Tropomyosin)

Background: Crustaceans and mollusks are a frequent cause of allergic reactions. The only major allergen identified in shrimp is the muscle protein tropomyosin; at least 80% of shrimp-allergic subjects react to tropomyosin. Furthermore, tropomyosin is an important allergen in other crustaceans such as lobsters, crabs and mollusks, as well as other arthropods such as house dust mites and cockroaches, and has been implied as the cause of clinical cross-sensitivity among invertebrates. In contrast, vertebrate tropomyosins are considered nonallergenic. Objective: The basis of the allergenicity of proteins has not yet been resolved. Thus, tropomyosin molecules provide an excellent opportunity to study the relationship between protein structure and allergenicity. The aim of the current study was to identify the IgE-binding regions of Pen a 1 and compare these regions with homologous sequences in other allergenic and nonallergenic tropomyosins. Methods: Forty-six overlapping peptides (length: 15 amino acids; offset: 6 amino acids) spanning the entire Pen a 1 molecule were synthesized and tested for IgE antibody reactivity with sera from 18 shrimp-allergic subjects to identify the IgE-binding regions of shrimp tropomyosin. Results: Based on the frequency and intensity of the IgE reactivities, five major IgE-binding regions were identified. All five major IgE-binding regions were 15-38 amino acids long. The major IgE-binding regions identified were: region 1: Pen a 1 (43-57); region 2: Pen a 1 (85-105); region 3: Pen a 1 (133-148); region 4: Pen a 1 (187-202), and region 5: Pen a 1 (247-284). In addition, 22 peptides were categorized as minor IgE-binding regions, and 12 peptides did not bind any IgE antibodies. No substantial differences in amino acid group composition in the five IgE-binding regions compared to the whole molecule were detected. Sequence identities and similarities of the Pen a 1 IgE-binding regions with homologous regions of allergenic arthropod tropomyosins were as high as 100%, whereas identities and similarities with homologous vertebrate sequences ranged from 36 to 76% and 53 to 85%, respectively. Conclusion: Five major IgE-binding regions of the allergenic shrimp tropomyosin, Pen a 1, were identified which are positioned at regular intervals of approximately 42 amino acids (7 heptads), suggesting a relationship with the repetitive coiled-coil structure of the tropomyosin molecule. The high degree of similarity between Pen a 1 IgE-binding regions and homologous sequences in invertebrate tropomyosins and the lower percentage of similarity with homologous regions of vertebrate tropomyosins supports a structural basis for cross-reactivity of allergenic tropomyosins.