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5 Physiological Significance of the Chromogranin-IP 3 R
Interaction
Despite the many similarities in physicochemical properties between CGA and
CGB and their near universal presence in a myriad of secretory cells (Helle 2000 ;
Montero-Hadjadje et al. 2008 ; Taupenot et al. 2003 ; Winkler and Fischer-Colbrie
1992 ; Bartolomucci et al. 2011 ), the subcellular localization of CGA is limited to
the cytoplasm, i.e., secretory granules, the ER, and the Golgi albeit most of it is in
acidic secretory granules. Hence, the interaction between CGA and the IP 3 Rs will
be the strongest and most stable inside secretory granules which maintain the IP 3 R/
Ca2+ channels in ordered and release-ready state (Yoo and Jeon 2000 ). However,
unlike the interaction of CGA with the IP 3 R/Ca2+ channels in secretory granules,
CGA in the ER is incapable of interacting with the IP 3 Rs due to its inability to bind
the IP 3 Rs at the physiological pH of the ER.
The failure of CGA to bind the IP 3 Rs directly at near physiological pH 7.5 is shown
with all three types of the IP 3 R (Yoo et al. 2001 ). As shown in Fig. 3 , CGA binds
directly with the IP 3 R at the intragranular pH 5.5, but it dissociates completely from
the IP 3 R at pH 7.5. Yet CGA interacts with CGB at pH 7.5, forming CGA- CGB het-
erodimer, thereby suggesting the presence of CGA-CGB heterodimers in the lumen of
the ER. Considering the fact that CGB interacts with all three IP 3 R types even at
pH 7.5, it is expected that the CGA-CGB heterodimers in the lumen of the ER stay
coupled to the IP 3 R/Ca2+ channels of the ER, and this is indeed the case as shown in
Fig. 3. As a result, the effect of CGA on the IP 3 R/Ca2+ channels will be exerted mainly
in secretory granules, and that in the ER will be only through its interaction with CGB,
which in turn interacts with the IP 3 R/Ca2+ channels (Fig. 3 ). Differing from CGA that
exists in the cytoplasm only, CGB localizes inside the nucleus as well. Moreover,
CGB interacts with the IP 3 Rs and activates the IP 3 R/Ca2+ channels regardless of the
different pH environment. Therefore, the IP 3 R/Ca2+ channel- activating effect of CGB
will be evident in both the cytoplasm and the nucleus (Huh et al. 2006a).
One of the major physiological effects of the strong interaction of CGB with the
IP 3 Rs is the prominent IP 3 R/Ca2+ channel-activating role of CGB, increasing the
Fig. 3 pH-dependent interaction of the IP 3 R with chromogranins. Purified IP 3 R1 (0.7–1.0 μg) was
reacted with GST-CGA, GST-CGB fusion proteins, and an equimolar mixture of the two (CGA/B)
at pH 5.5 and 7.5 in the presence of 2 mM Ca2+. The bound IP 3 R1 was separated on a 7.5% SDS
gel and analyzed by immunoblot
S. H. Yo o