Presence of aquaporin and V-ATPase on the contractile vacuole of Amoeba proteus.
BACKGROUND
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.
Nishihara
Eri
E
Department of Life Science, Graduate School of Life Science, University of Hyogo, Harima Science Park City, Hyogo 678-1297, Japan. eniskid@tmd.ac.jp
Yokota
Etsuo
E
Tazaki
Akira
A
Orii
Hidefumi
H
Katsuhara
Maki
M
Kataoka
Kensuke
K
Igarashi
Hisako
H
Moriyama
Yoshinori
Y
Shimmen
Teruo
T
Sonobe
Seiji
S
D020816:Amino Acid Motifs / Q000502:physiology; D000595:Amino Acid Sequence; D000656:Amoeba / Q000378:metabolism* / Q000648:ultrastructure; D000818:Animals; D020346:Aquaporins / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D001483:Base Sequence; D002463:Cell Membrane Permeability / Q000502:physiology; D003285:Contractile Proteins / Q000378:metabolism; D007425:Intracellular Membranes / Q000378:metabolism / Q000648:ultrastructure; D008969:Molecular Sequence Data; D017434:Protein Structure, Tertiary / Q000502:physiology; D015800:Protozoan Proteins / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism; D025262:Vacuolar Proton-Translocating ATPases / Q000378:metabolism*; D014617:Vacuoles / Q000378:metabolism* / Q000648:ultrastructure; D014882:Water-Electrolyte Balance / Q000502:physiology
D020346:Aquaporins; D003285:Contractile Proteins; D015800:Protozoan Proteins; D025262:Vacuolar Proton-Translocating ATPases