Localization and possible function of aquaporin NIP1;1

NIP1;1 (Nod 26 like Intrinsic Protein 1;1) is an aquaglyceroporin involved in transport and tolerance to As(III) and regulated negatively by ABA, NaCl, dark and dry stress. It is a MIP (Major Intrinsic Proteins) as many others transmembrane proteins collectively named aquaporins, involved in regulation of the cell’s homeostasis. The cDNAs cluster into three groups. The tonoplast aquaporins (TIPs) and the plasma membrane aquaporins (PIPs) form large groups, whereas NIPs remain few, distinct from both TIPs and PIPs. Aquaporins have a predicted topology with six membrane-spanning domains connected by five loops and short N-terminal and C-terminal domains in the cytoplasm. NIP1;1 has a hydrophilic N-terminal tail with approximately the same charges found in PIPs but the C-terminal tail has two negative charges and is closer to plant TIPs. Unique to the NIP1;1 subgroup is a highly conserved Cys residue in, or close to, the end of the first putative transmembrane domain. It is interesting that the aminoacidic motif NPA (asparagine-proline-alanine), characteristic for the MIP family, is not fully conserved in NIP1;1. The NPA motif is replaced by NPG (Asn-Pro-Gly) in NIP1;1. Kamiya and co-workers reported to have localized GFP-NIP1;1 to the plasma membrane but their images could be questioned. In fact no co-localization experiments were performed and the distribution of fluorescence was not sharp and limited to the plasma membrane but distributed in the cell protoplasma (Kamiya et al., 2009). We described for AtNIP1;1 a more complex distribution using an identical GFP-tagged chimera. We observed GFP-NIP1;1 in the endoplasmic reticulum and in well defined donut-like structures that could be described as karmellae but of which we started the characterization. A bioinformatics analysis on network interaction data, evidenced a direct interaction between NIP1;1 and some SNAREs among which is found SYP51, a Qc SNARE involved in vacuolar transport, exocytosis and endocytosis (De Benedictis et al., 2013). The closely related Qc SNARE SYP52, with an high level of homology to SYP51, did not interact with NIP1;1. We have investigated in vivo if NIP 1.1 interact with SYP51/52 trough ratiometric bimolecular fluorescence complementation (rBiFC) (Grefen and Blatt, 2012) using appropriate controls. The experiments confirmed the specific interaction between NIP1;1 and SYP51. Essential bibliography: De Benedictis, M., Bleve, G., Faraco, M., Stigliano, E., Grieco, F., Piro, G., Dalessandro, G., and Di Sansebastiano, G. Pietro (2013). AtSYP51/52 functions diverge in the post-Golgi traffic and differently affect vacuolar sorting. Mol. Plant 6: 916–30. Grefen, C. and Blatt, M.R. (2012). A 2in1 cloning system enables ratiometric bimolecular fluorescence complementation (rBiFC). Biotechniques 53: 311–14. Kamiya, T., Tanaka, M., Mitani, N., Ma, J.F., Maeshima, M., and Fujiwara, T. (2009). NIP1;1, an aquaporin homolog, determines the arsenite sensitivity of Arabidopsis thaliana. J. Biol. Chem. 284: 2114–20.