Presence of aquaporin and V-ATPase on the contractile vacuole of Amoeba proteus

• Published in: Biology of the cell 2008-03, Vol.100 (3), p.179-188
• Main Authors: Nishihara, Eri, Yokota, Etsuo, Tazaki, Akira, Orii, Hidefumi, Katsuhara, Maki, Kataoka, Kensuke, Igarashi, Hisako, Moriyama, Yoshinori, Shimmen, Teruo, Sonobe, Seiji
• Format: Article
• Language: English
• Subjects: Amino Acid Motifs - physiology; Amino Acid Sequence; Amoeba; Amoeba - metabolism; Amoeba - ultrastructure; Amoeba proteus; Animals; aquaporin; Aquaporins - genetics; Aquaporins - isolation & purification; Aquaporins - metabolism; Base Sequence; Cell Membrane Permeability - physiology; Contractile Proteins - metabolism; contractile vacuole; Intracellular Membranes - metabolism; Intracellular Membranes - ultrastructure; Molecular Sequence Data; osmoregulation; Protein Structure, Tertiary - physiology; Proteus; Protozoan Proteins - genetics; Protozoan Proteins - isolation & purification; Protozoan Proteins - metabolism; vacuolar H+-ATPase (V-ATPase); Vacuolar Proton-Translocating ATPases - metabolism; Vacuoles - metabolism; Vacuoles - ultrastructure; Water-Electrolyte Balance - physiology; Xenopus
• ISSN: 0248-4900; 1768-322X
• DOI: 10.1042/BC20070091

Background information. The results of water permeability measurements suggest the presence of an AQP (aquaporin) in the membrane of the CV (contractile vacuole) in Amoeba proteus [Nishihara, Shimmen and Sonobe (2004) Cell Struct. Funct. 29, 85–90]. Results. In the present study, we cloned an AQP gene from A. proteus [ApAQP (A. proteus AQP)] that encodes a 295‐amino‐acid protein. The protein has six putative TMs (transmembrane domains) and two NPA (Asn‐Pro‐Ala) motifs, which are conserved among various AQPs and are thought to be involved in the formation of water channels that span the lipid bilayer. Using Xenopus oocytes, we have demonstrated that the ApAQP protein product can function as a water channel. Immunofluorescence microscopy with anti‐ApAQP antibody revealed that ApAQP is detected on the CV membrane and on the vesicles around the CV. The presence of V‐ATPase (vacuolar H+‐ATPase) on the vesicle membrane around the CV was also detected. Conclusions. Our data on ApAQP allow us to provide the first informed explanation of the high water permeability of the CV membrane in amoeba. Moreover, the results suggest that vesicles possessing V‐ATPase are involved in generating an osmotic gradient. Based on our findings, we propose a new hypothesis for the mechanism of CV function.