1.A.8.11.4
Plasma membrane intrinsic protein 2a (forms active heterotetramers with PIP1;1 (TC# 1.A.8.11.3); down regulated under drought stress (Najafabadi et al., 2008). Transports H₂O₂ (Dynowski et al., 2008). The Mesembryanthemum crystallinum PIP2;1 orthologue is an aquaporin impermeable to urea and glycerol. It is positively regulated by PKA- and PKC- mediated phosphorylation (Amezcua-Romero et al., 2010). PIP1;1 and PIP2;2 (Q9ATM8) co-expression modulates the membrane water permeability in the halophyte Beta vulgaris storage root through a pH regulatory response, enhancing membrane versatility to adjust its water transfer capacity (Bellati et al., 2010). The wheat orthologue has been described (Ayadi et al., 2011).  Inter-TMS interactions occurring both within and between monomers play crucial roles in tetramer formation, and assembly of tetramers is critical for their trafficking from the ER to the plasma membrane as well as water permeability (Yoo et al. 2016).  This protein as well as 1.A.8.11.6 is possibly orthologous to spinach PIP1;2 for which the crystal structure is available (PDP# 4JC6) (Berny et al. 2016).  Plays a role in drought and salt tolerance (Wang et al. 2015). PIP-type aqauporins may also transport CO₂, boric acid, glycerol, arsenic and Na⁺ (Byrt et al. 2017). TMS2 and TMS3 are necessary and sufficient in AtPIP2 for its PM localization (Wang et al. 2019).  Arabidopsis thaliana plasma membrane intrinsic protein (AtPIP2;1) is implicated in a salinity conditional influence on seed germination (Hoai et al., 2023; PMID 37277902).