A role of the C-terminus of aquaporin 4 in its membrane expression in cultured astrocytes.

Abstract

Aquaporin 4 (AQP4) is a predominant water channel protein in mammalian brains, which is localized in the astrocyte plasma membrane. Membrane targeting of AQP4 is essential to perform its function. The mechanism(s) of membrane targeting is not clear in astrocytes. We investigated the role of the C-terminus of AQP4 (short isoform) in its membrane targeting by an expression study of C-terminal mutants of AQP4 in cultured astrocytes. The deletion of 26 C-terminal residues of AQP4 (AQP4[Delta276-301aa]) results in the intracellular localization of the protein. However, smaller deletions than 21 C-terminal residues did not alter its plasma membrane localization. These results suggest that C-terminal residues between Val(276) and Ile(280) play an important role in the expression of AQP4 in the plasma membrane. However, the plasma membrane localization of the AQP4(A(276)AAAA(280)) mutant (alanine substitution of Val(276)-Ile(280) of AQP4) suggests that another signal for membrane targeting exists in the C-terminus of AQP4. The deletion or point mutations of the PDZ binding motif of the AQP4(A(276)AAAA(280)) mutant resulted in the intracellular localization of the proteins. These results suggest that the PDZ binding motif may also be involved in the membrane targeting of AQP4. We found that the C-terminal sequence of AQP4 contains two important signals for membrane expression of AQP4 in cultured astrocytes. One is a hydrophobic domain and the other is a PDZ binding motif that exists in the C-terminus.