The Journal of General Physiology
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Published online 14 February 2005 doi:10.1085/jgp.200409230
The Rockefeller University Press, 0022-1295 $8.00
JGP, Volume 125, Number 3, 327-334
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PKC-induced Sensitization of Ca2+-dependent Exocytosis Is Mediated by Reducing the Ca2+ Cooperativity in Pituitary Gonadotropes

Hua Yang1, Huisheng Liu1, Zhitao Hu1, Hongliang Zhu1, and Tao Xu1,2

1 Institute of Biophysics and Biochemistry, School of Life Science and Technology, Huazhong University of Science and Technology, Wuhan 430074, P.R. China
2 National Laboratory of Biomacromolecules, Institute of Biophysics, Chinese Academy of Sciences, Beijing 100101, P.R. China

Correspondence to Tao Xu: xutao{at}ibp.ac.cn

The highly cooperative nature of Ca2+-dependent exocytosis is very important for the precise regulation of transmitter release. It is not known whether the number of binding sites on the Ca2+ sensor can be modulated or not. We have previously reported that protein kinase C (PKC) activation sensitizes the Ca2+ sensor for exocytosis in pituitary gonadotropes. To further unravel the underlying mechanism of how the Ca2+ sensor is modulated by protein phosphorylation, we have performed kinetic modeling of the exocytotic burst and investigated how the kinetic parameters of Ca2+-triggered fusion are affected by PKC activation. We propose that PKC sensitizes exocytosis by reducing the number of calcium binding sites on the Ca2+ sensor (from three to two) without significantly altering the Ca2+-binding kinetics. The reduction in the number of Ca2+-binding steps lowers the threshold for release and up-regulates release of fusion-competent vesicles distant from Ca2+ channels.

Key Words: exocytosis • kinetic modeling • Ca2+ dependency • fusion • protein phosphorylation

H. Yang and H. Liu contributed equally to this work.

Abbreviations used in this paper: DMN, DM-nitrophen; HCSP, highly Ca2+-sensitive pool; KS, Kolmogorov-Smirnov; NP-EGTA, nitrophenyl-EGTA.


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