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August 18, 2006

More responsibilities for a busy pathway

New findings reveal a previously unknown link connecting a major cellular signaling pathway with the regulation of physiological pH levels

Figure 1: IRBIT is bound to the IP3 binding core of the cytoplasmic domain of IP3R under normal conditions (left). Binding of IP3 to the receptor causes it to release (right), after which IRBIT associates with pNBC1 to stimulate ion transport across the plasma membrane (top). (Ca2+ = calcium ion; CO2 = carbon dioxide; H2O = water; H+ = hydrogen; HCO3- = hydrogen carbonate; yellow circle = phosphate).

The release of calcium ions into the cytoplasm from storage organelles is mediated in part by the binding of inositol 1,4,5-trisphosphate (IP3) to its receptor (IP3R). Calcium release triggered by IP3 is known to play an important role in a wide variety of essential cellular activities.

A team of neurobiologists led by Katsuhiko Mikoshiba of RIKEN’s Brain Science Institute, in Wako, identified IP3R binding protein called IRBIT. IRBIT binds to IP3R under normal physiological conditions but dissociates from the receptor in the presence of elevated concentrations of IP31,2. This led them to believe that IRBIT could be mediating the transmission of IP3-induced signals to other targets.

Now, Mikoshiba and his colleagues from RIKEN, the University of Tokyo and the Japan Science and Technology Corporation report in the Proceedings of the National Academy of Sciences3 that they have identified a target for IRBIT binding and activation. The target is a protein called NBC1, which is involved in moving ions across the plasma membrane. NBC1 exists in two major forms: one found predominantly in the kidney (kNBC1), and the other expressed primarily in the pancreas (pNBC1). According to Mikoshiba, improper function of pNBC1 can have severe physiological consequences. “pNBC1 is involved in acid-base balance. If the activity is low, the balance shifts to acidosis, which results in cataracts and glaucoma,” he says. “It is also expressed in the nervous system... [and] abnormality of pNBC1 results in mental retardation and low growth rate.”

Protein binding experiments showed that IRBIT associates specifically with pNBC1 but not kNBC1, which differs only by a short stretch of amino acids at one end of the protein. Additional experiments performed in frog oocytes showed that pNBC1 appears to be dependent on coexpression of IRBIT in order to function properly, suggesting that the latter protein acts as an activator of pNBC1, directly linking cotransporter activity to IP3 signaling (Fig. 1).

This finding came as somewhat of a surprise to Mikoshiba’s group. “This is the first report to directly link IP3 signaling to pNBC1 cotransporter activity,” he says. “It is exciting to know that the role of IP3 is not only to release calcium ions, but also to release IRBIT and work on pNBC1, which regulates pH balance.” They believe that IRBIT regulation may play an important part in controlling proper pNBC1 function, and are now attempting to identify the detailed mechanism by which IRBIT is released from IP3R and binds to pNBC1.

Related highlight: Molecular mimic sets the tone for calcium release

References

  1. Ando, H., Mizutani, A., Matsu-ura, T., & Mikoshiba, K. IRBIT, a novel inositol 1,4,5-trisphosphate (IP3) receptor-binding protein, is released from the IP3 receptor upon IP3 binding to the receptor. Journal of Biological Chemistry 278 (12), 10602–10612 (2003). || (Link)
  2. Ando, H., Mizutani, A., Tsuzurugi, D., Michikawa, T., & Mikoshiba, K. IRBIT suppresses IP3 receptor activity by competing with IP3 for the common binding site on IP3 receptor in a phosphorylation-dependent manner. Molecular Cell 22, 1–12 (2006). || (Link)
  3. Shirakabe, K., Priori, G., Yamada, H., Ando, H., Horita, S., Fujita, T. Fujimoto I, Mizutani, A., Seki, G., & Mikoshiba, K. IRBIT, an inositol 1,4,5-trisphosphate receptor-binding protein, specifically binds to and activates pancreas-type Na+/HCO3- cotransporter 1 (pNBC1). Proceedings of the National Academy of Sciences USA103 (25), 9542–9547 (2006). || (Link)