Loss of a member of the aquaporin gene family, aqpA affects spore dormancy in Dictyostelium

We isolated and characterized a gene from Dictyostelium discoideum, which encodes a protein of 279 amino acids (30.6 kDa) containing six transmembrane domains with two highly conserved motifs of asparagine-proline-alanine (NPA) found in the aquaporin family of water-channel proteins, although the second motif of the protein has been modified into NPV (asparagine proline-valine), The deduced amino acid sequence of the gene, which we have named aqpA, is 39% identical to D. discoideum WacA, 26% identical to human Ayp5, 26% identical to Oryza savtiva PIP2a, 25% identical to yeast Aqyl and 24% identical to E. coli AqpZ. Southern analyses indicated that aqpA is present as a single copy in the genome. Northern blot analysis showed that the developmentally regulated 1 kb mRNA transcript first appears at the tight mound stage (12 h), and is abundant in fingers (16 h) and late culminants (20 h). In-situ hybridization of slugs revealed that aqpA mRNA accumulated in cells of the prespore region but not in those of the prestalk region. Disruption of aqpA by homologous recombination did not significantly affect growth or developmental morphogenesis. Although mutant spores were viable, when assayed soon after encapsulation, they became permeable to propidium iodide and lost viability after a week on the top of a fruiting body. Thus, AqpA is essential to maintain spore dormancy perhaps through the regulation of water flow. (C) 2000 Elsevier Science B.V. All rights reserved.