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The mechanism of action of GTP on Ca2+ efflux from rat liver microsomal vesicles

A P Dawson1, G Hills, J G Comerford

  • 1School of Biological Sciences, University of East Anglia, Norwich, U.K.

The Biochemical Journal
|May 15, 1987
PubMed

Insights

Guanosine triphosphate (GTP) triggers calcium (Ca2+) release and enhances inositol trisphosphate (IP3)-induced release from vesicles, likely through a GTP-dependent vesicle fusion mechanism.

Area of Science:

  • Cell biology
  • Biochemistry
  • Membrane dynamics

Background:

  • Guanosine triphosphate (GTP) plays a role in cellular signaling pathways.
  • Inositol 1,4,5-triphosphate (IP3) is a key second messenger involved in intracellular calcium (Ca2+) release.
  • Microsomal vesicles are crucial for calcium storage and release within cells.

Purpose of the Study:

  • To investigate the mechanism by which GTP influences Ca2+ release from microsomal vesicles.
  • To determine the role of GTP in modulating IP3-promoted Ca2+ release.
  • To explore the structural changes in vesicles induced by GTP.

Main Methods:

  • Utilized guanosine 5'-[gamma-thio]triphosphate (GTP[S]) as a non-hydrolyzable GTP analog.
  • Performed Ca2+ efflux assays on preloaded microsomal vesicles.
  • Employed electron microscopy to visualize vesicle structure.
  • Monitored light-scattering changes to detect fusion events.

Main Results:

  • GTP[S] added before GTP inhibited GTP-induced Ca2+ efflux and enhancement of IP3-promoted release.
  • GTP's effect on Ca2+ release persisted after vesicle washing, independent of poly(ethylene glycol).
  • Electron microscopy revealed GTP-induced fusion of smaller vesicles into larger structures.
  • Light scattering confirmed fusion during GTP treatment.

Conclusions:

  • GTP-dependent vesicle fusion is the likely mechanism underlying both GTP-promoted Ca2+ efflux and the enhancement of IP3-mediated Ca2+ release.
  • GTP binding induces significant structural changes in microsomal vesicles, leading to fusion.
  • These findings elucidate a novel role for GTP in regulating intracellular calcium dynamics through membrane fusion.

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