FUNCTION AND REGULATION OF SEED AQUAPORINS

The discovery of water channel proteins named aquaporins has shed new light on the molecular mechanisms of transmembrane water transport in higher plants, As with their animal counterparts, plant aquaporins bel ong to the large MIP family of transmembrane channels. An increasing n umber of aquaporins is now being identified on both the vacuolar and p lasma membranes of plant cells, but their integrated function remains unclear. Aquaporin alpha-TIP is specifically expressed in the membrane of protein storage vacuoles in seeds of many plant species, alpha-TIP was previously shown to undergo phosphorylation in bean seeds. The fu nctional significance of this process was further investigated after h eterologous expression of the protein in Xenopus oocytes. Using site-d irected mutagenesis of alpha-TIP and in vitro and in vivo phosphorylat ion by animal cAMP-dependent protein kinase, it is shown that, in oocy tes, direct phosphorylation of alpha-TIP occurs at three distinct site s and stimulates its water channel activity. In addition to aquaporin phosphorylation, other mechanisms that target aquaporin function are u sed by living cells to regulate their membrane water permeability. The se are the fine control of aquaporin gene expression and, in animal ce lls only, the regulated trafficking of water channel-containing vesicl es, The present work and studies by others on the phosphorylation of n odulin-26, an ion channel protein homologous to alpha-TIP, provide nov el insights into the mechanisms of plant membrane protein regulation. These studies might help identifying and characterizing novel membrane -bound protein kinases and phosphatases. Finally, an integrated functi on for seed vacuolar aquaporins is discussed. During germination, the rehydration of seed cells, the drastic changes in vacuole morphology, the breakdown and the mobilization of storage products from the vacuol e may create osmotic perturbations in the cytoplasm. The fine tuning o f TIP aquaporin activity may help control the kinetics and amplitude o f osmotic water flows across the tonoplast to achieve proper cytoplasm osmoregulation and control of vacuolar volume.