Olive pollen profilin (Ole e 2 allergen) co-localizes with highly active areas of the actin cytoskeleton and is released to the culture medium during in vitro pollen germination

Pollen allergens offer a dual perspective of study: some of them are considered key proteins for pollen physiology, but they are also able to trigger allergy symptoms in susceptible humans after coming in contact with their tissues. Profilin (Ole e 2 allergen) has been characterized, to some extent, as one of the major allergens from Olea europaea L. pollen, a highly allergenic species in the Mediterranean countries. In order to obtain clues regarding the biological role of this protein, we have analyzed both its cellular localization and the organization of actin throughout pollen hydration and early pollen tube germination. The localization of the cited proteins was visualized by confocal laser scanning microscopy immunofluorescence using different antibodies. Upon pollen hydration and pollen germination, a massive presence of profilin was detected close to the site of pollen tube emergence, forming a ring-like structure around the 'effective' apertural region. Profilin was also detected in the pollen exine of the germinating pollen grains and in the germination medium. After using a permeabilization-enhanced protocol for immunolocalization, profilin was also localized in the cytoplasm of the pollen tube, particularly at both the proximal and apical ends. Noticeable accumulations of actin were observed in the cytoplasm of the pollen tube; particularly, in both the apical region and the area immediately close to the aperture. Actin filaments were not observed, probably due to the need of further enhanced fixation procedures. The ultrastructural localization of profilin showed the presence of the protein in the cytoplasm of both the mature pollen grain and the pollen tube. The results shown here could be interpreted as signs of a massive dissociation of the actin-profilin complexes, mobilization of actin monomers, and therefore, an intense activity of the actin cytoskeleton. The extensive release of allergenic proteins from the pollen grain into the surrounding aqueous media, as described here for profilin, may help us to understand the mechanisms by which these allergens might come in contact with the human mucosa, therefore triggering the symptoms of allergy.