Three amino-acid sequences regulate the expression level of aquaporin-4

Abstract

Aquaporin-4 (AQP4) is a water cannel playing a role in water transport and homeostasis of the brain. Previously, we observed that deletion of the C-terminal domain causes degradation of AQP4. In this report, we identified three amino-acid sequences that regulate the expression level of AQP4. First, deleting C-terminal 10 amino acids (Asp 314 -Val 323 ) greatly reduced the level of AQP4. Substitutions of Ala for Asp 314 and/or Glu 318 mimicked this effect, suggesting that two acidic amino acids in this region is important to prevent AQP4 from degradation. Second, this reduction of AQP4 was rescued when the C-terminal domain was deleted more than 43 amino acids, suggesting that the region between Val 280 and Lys 313 contains a signal for the degradation. Substitution of Phe for Tyr 277 or Arg for Val 280 increased the level of AQP4 lacking C-terminal 42 amino acids (Δ282-323), suggesting that a tyrosine-based endocytic motif (YXXΦ) is involved in the degradation of AQP4. Finally, deletion between Lys 259 and Ala 270 increased the level of AQP4. In contrast to the disruption of the putative YXXΦmotif, the 12-amino-acid deletion could not rescue AQP4 Δ282-323 from degradation, indicating that the deletion increased the level of AQP4 with a different mechanism from YXXΦmotif mutants.