Molecular metamorphosis in polcalcin allergens by EF‐hand rearrangements and domain swapping

Abstract

Polcalcins such as Bet v 4 and Phl p 7 are pollen allergens that are constructed from EF‐hand motifs, which are very common and well characterized helix–loop–helix motifs with calcium‐binding functions, as elementary building blocks. Being members of an exceptionally well‐characterized protein superfamily, these allergens highlight the fundamental challenge in explaining what features distinguish allergens from nonallergenic proteins. We found that Bet v 4 and Phl p 7 undergo oligomerization transitions with characteristics that are markedly different from those typically found in proteins: transitions from monomers to dimers and to distinct higher oligomers can be induced by increasing temperature; similarly, low concentrations of destabilizing agents, e.g. SDS, induce oligomerization transitions of Bet v 4. The changes in the quaternary structure, termed molecular metamorphosis, are induced and controlled by a combination of EF‐hand rearrangements and domain swapping rather than by the classical law of mass action. Using an EF‐hand‐pairing model, we provide a two‐step model that consistently explains and substantiates the observed metamorphosis. Moreover, the unusual oligomerization behavior suggests a straightforward explanation of how allergens can accomplish the crosslinking of IgE on mast cells, a hallmark of allergens. Structured digital abstract http://mint.bio.uniroma2.it/mint/search/interaction.do?interactionAc=MINT-7718612: Bet v 4 (uniprotkb:http://www.uniprot.org/uniprot/Q39419) and Bet v 4 (uniprotkb:http://www.uniprot.org/uniprot/Q39419) bind (http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0407) by molecular sieving (http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0071) http://mint.bio.uniroma2.it/mint/search/interaction.do?interactionAc=MINT-7718648: Phl p 7 (uniprotkb:http://www.uniprot.org/uniprot/O82040) and Phl p 7 (uniprotkb:http://www.uniprot.org/uniprot/O82040) bind (http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0407) by molecular sieving (http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0071)

EAACI Molecular Allergology User's Guide

“…Polcalcins are 9‐kDa calcium‐binding pollen proteins of unknown biologic function. While regular polcalcins contain two EF‐hand domains, several polcalcin‐related allergens with three or four EF‐hand domains were described . Polcalcins were shown to be minor albeit highly cross‐reactive allergens identified in pollen from diverse plant families (Fig.…”

ROS in tobacco stigma exudate affect pollen proteome and provoke membrane hyperpolarization

“…Polcalcins are pollen-specific proteins containing two EF-hands highly conserved in a variety of dicot and monocot species (Rozwadowski et al 1999). Polcalcins are mainly studied as strong human allergens (Magler et al 2010), but immunolocalization in certain model species showed that they are abundant cytosolic low-Mr proteins located at or near the pollen tube surface (Rozwadowski et al 1999). Polcalcins bind calcium and undergo changes in conformation, thus, should display regulatory function and perhaps is involved in signal transduction pathways (Ledesma et al 1998).…”

Interplay between Affinity and Valency in Effector Cell Degranulation: A Model System with Polcalcin Allergens and Human Patient–Derived IgE Antibodies

“…3). We have also produced monomeric polcalcin mutants stabilized by an internal disulfide bridge, which stabilizes the monomer conformation, as described by Magler et al (41). When RBL-SX38 cells were sensitized with either HAPPIE1 or HAPPIE2, no significant degranulation was detected when stimulated either wild-type monomer or disulfidestabilized monomeric allergens (Fig.…”

“…The family of polcalcin proteins have been shown to exist both as monomers and domain-swapped homodimers (41). Recombinantly expressed polcalcins can form monomers and intertwined dimers, and these oligomeric states can be separated using size-exclusion chromatography during the purification process (Supplemental Fig.…”