Identification of a Multiprotein "Motor" Complex Binding to Water Channel Aquaporin-2

Line
Title: Identification of a Multiprotein "Motor" Complex Binding to Water Channel Aquaporin-2
Authors: Noda, Yumi; Horikawa, Saburo; Katayama, Yoshifumi; Sasaki, Sei
Publisher: Biochemical and Biophysical Research Communications
Date Published: May 20, 2005
Reference Number: 687
Line
Targeted positioning of water channel aquaporin-2 (AQP2) strictly regulates body water homeostasis. Trafficking of AQP2 to the apical membrane is critical to the reabsorption of water in renal collecting ducts. Recently, we have identified for the first time proteins which directly bind to AQP2: SPA-1, a GTPase-activating protein for Rap1, and cytoskeletal protein actin. Based on these findings, we have speculated the existence of a multiprotein complex which includes AQP2, SPA-1, and actin, for providing the mechanism which generates force and motion in AQP2 trafficking. To clarify the proteins comprising the complex, a large amount of AQP2-associated protein complex was isolated from the extract of rat kidney papilla using immunoaffinity column coupled with anti-AQP2 antibody and was analyzed by matrix-assisted laser desorption-ionization time-of-flight mass spectrometry (MALDI-TOF MS). In addition to SPA-1 and actin, 11 proteins were identified using this method: ionized calcium binding adapter molecule 2, myosin regulatory light chain smooth muscle isoforms 2-A and 2-B, a-tropomyosin 5b, annexin A2 and A6, scinderin, gelsolin, a-actinin 4, a-II spectrin, and myosin heavy chain nonmuscle type A. Our findings show for the first time an AQP2-binding multiprotein "force generator" complex. This multiprotein complex may provide the machinery of driving AQP2 movement.

This translation by the NDI Foundation is to assist the lay reader. To provide a clear, accessible interpretation of the original article, we eliminated or simplified some technical detail and complicated scientific language. We concentrated our translation on those aspects of the article dealing directly with NDI. The NDI Foundation thanks the researchers for their work toward understanding and more effectively treating this disorder.
© Copyright NDI Foundation 2007 (JC)

When the hormone vasopressin (AVP) binds with the vasopressin-2 receptor (V2R), it initiates a molecular sequence that stimulates the protein, aquaporin-2 (AQP2) to move from the interior of the kidney collecting duct principal cell to the cell’s membrane. This process plays an essential role in the kidney’s ability to concentrate urine and balance body water. How the AQP2 makes this journey from cell interior to cell membrane was made clearer by the recent research of Noda, et al.

In that research, the research team identified two proteins that bind directly to AQP2: SPA-1 and actin. This finding led the researchers to speculate that these two proteins and AQP2 were part of a multi-protein complex that generates the force and motion necessary for AQP2 to make its trip.

In their most recent research, Noda, et al., analyzed a large amount of rat kidney to see if they could identify the proteins involved in the AQP2-associated protein complex. They found: ionized calcium binding adapter molecule 2, myosin regulatory light chain smooth muscle isoforms 2-A and 2-B, a-tropomyosin 5b, annexin A2 and A6, scinderin, gelsolin, a-actinin 4, a-II spectrin, and myosin heavy chain nonmuscle type A.

Since all the proteins identified are capable of binding with actin, the researchers speculate that these proteins may indirectly associate with AQP2 via actin. The researchers further speculate that these proteins form a complex that may provide this “motor” that drives the AQP2 to the cell membrane. They plan further research to clarify this.