Water Channel Properties of Major Intrinsic Protein of Lens

• Published in: The Journal of biological chemistry Vol. 270; no. 15; pp. 9010 - 9016
• Main Authors: Mulders, Sabine M., Preston, Gregory M., Deen, Peter M.T., Guggino, William B., van Os, Carel H., Agre, Peter
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 14-04-1995; American Society for Biochemistry and Molecular Biology
• Subjects: Amino Acid Sequence; Animals; Aquaporin 1; Aquaporins; Base Sequence; Cloning, Molecular; DNA Primers; Eye Proteins - chemistry; Eye Proteins - genetics; Eye Proteins - metabolism; Ion Channels - chemistry; Ion Channels - genetics; Ion Channels - metabolism; Membrane Glycoproteins; Microscopy, Fluorescence; Molecular Sequence Data; Water; Xenopus
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.270.15.9010

The functions of major intrinsic protein (MIP) of lens are still unresolved; however the sequence homology with channel-forming integral membrane protein (CHIP) and other Aquaporins suggests that MIP is a water channel. Immunolocalizations confirmed that Xenopus oocytes injected with bovine MIP cRNA express the protein and target it to the plasma membrane. Control oocytes or oocytes expressing MIP or CHIP exhibited small, equivalent membrane currents that could be reversibly increased by osmotic swelling. When compared with water-injected control oocytes, the coefficient of osmotic water permeability (Pf) of MIP oocytes was increased 4-5-fold with a low Arrhenius activation energy, while the Pfof CHIP oocytes increased > 30-fold. To identify structures responsible for these differences in Pf, recombinant MIP proteins were expressed. Analysis of MIP-CHIP chimeric proteins revealed that the 4-kDa cytoplasmic domain of MIP did not behave as a negative regulator. Individual residues in MIP were replaced by residues conserved among the Aquaporins, and introduction of a proline in the 5th transmembrane domain of MIP raised the Pfby 50%. Thus oocytes expressing MIP failed to exhibit ion channel activity and consistently exhibited water transport by a facilitated pathway that was qualitatively similar to the Aquaporins but of lesser magnitude. We conclude that MIP functions as an Aquaporin in lens, but the protein may also have other essential functions.