Invertase inhibitor (Probable). Monomer. After 5 min in germination medium, detected as a soluble protein and released from apertural and non-apertural regions of pollen grain wall. In mature, non-hydrated pollen grains, predominantly associated with the endoplasmic reticulum (ER) concentric cisternae, which are situated between the vegetative nucleus and the generative cell. After 5, 15 and 30 min as well as 2 h of hydration, localization to this ER region is noticeably decreased. Expressed in pollen (at protein level) (PubMed:12859456, PubMed:11906336, PubMed:16179793, Ref.4, PubMed:17177673). Expressed in stem, but not leaves (at protein level) (PubMed:12859456). Expressed in pollen (PubMed:12859456). Not glycosylated. Causes an allergic reaction in human. Binds to IgE of patients allergic to London plane tree pollen (PubMed:12859456, PubMed:11906336, PubMed:16179793, PubMed:17177673). Binds to IgE in 83% of the 42 patients tested (PubMed:12859456). Binds to IgE in 92% of the 12 patients tested (PubMed:11906336). Binds to IgE in 79% of the 47 Spanish patients tested (PubMed:16179793). Binds to IgE in 75% of the 26 patients tested (PubMed:17177673). Binds to IgE in 83% of the 18 patients tested allergic London plane tree pollen as well as other pollens including grass and olive (PubMed:11906336). The developed two-site sandwich ELISA assay to quantify this protein in P.acerifolia pollen extracts could be useful for the quality control of allergen preparations intended for clinical use (PubMed:16179793). Could be used together with allergen Pla a 2 for a reliable diagnosis of London plane tree (P.acerifolia) pollen allergy by skin prick test (SPT) in most patients of the cohort of 47 tested with a sensitivity of over 90% and a specificity of 100% (PubMed:17177673). Belongs to the PMEI family.