Channel protein in tonoplast. These proteins may allow the diffusion of amino acids and/or peptides from the vacuolar compartment to the cytoplasm. Tonoplast. Roots. Aquaporins contain two tandem repeats each containing three membrane-spanning domains and a pore-forming loop with the signature motif Asn-Pro-Ala (NPA). Belongs to the MIP/aquaporin (TC 1.A.8) family. TIP (TC 1.A.8.10) subfamily.