Physiological concentrations of K+ inhibit cytochrome c-dependent formation of the apoptosome.
In many forms of apoptosis, cytochrome c released from mitochondria induces the oligomerization of Apaf-1 to form a caspase-activating apoptosome complex. Activation of lysates in vitro with dATP and cytochrome c results in the formation of an active caspase-processing approximately 700-kDa apoptosome complex, which predominates in apoptotic cells, and a relatively inactive approximately 1.4-MDa complex. We now demonstrate that assembly of the active complex is suppressed by normal intracellular concentrations of K(+). Using a defined apoptosome reconstitution system with recombinant Apaf-1 and cytochrome c, K(+) also inhibits caspase activation by abrogating Apaf-1 oligomerization and apoptosome assembly. Once assembled, the apoptosome is relatively insensitive to the effects of ionic strength and processes/activates effector caspases. The inhibitory effects of K(+) on apoptosome formation are antagonized in a concentration-dependent manner by cytochrome c. These studies support the hypothesis that the normal intracellular concentrations of K(+) act to safeguard the cell against inappropriate formation of the apoptosome complex, caused by the inadvertent release of small amounts of cytochrome c. Thus, the assembly and activation of the apoptosome complex in the cell requires the rapid and extensive release of cytochrome c to overcome the inhibitory effects of normal intracellular concentrations of K(+).
Cain
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K
MRC Toxicology Unit, Hodgkin Bldg., University of Leicester, P.O. Box 138, Lancaster Rd., Leicester LE1 9HN, United Kingdom. kc5@le.ac.uk
Langlais
C
C
Sun
X M
XM
Brown
D G
DG
Cohen
G M
GM
D017209:Apoptosis; D053477:Apoptotic Protease-Activating Factor 1; D020169:Caspases / Q000502:physiology; D002460:Cell Line; D003574:Cytochrome c Group / Q000502:physiology*; D003838:Deoxyadenine Nucleotides / Q000494:pharmacology; D006801:Humans; D008970:Molecular Weight; D009994:Osmolar Concentration; D011188:Potassium / Q000494:pharmacology*; D011506:Proteins / Q000737:chemistry*; D011994:Recombinant Proteins / Q000096:biosynthesis
C506592:APAF1 protein, human; D053477:Apoptotic Protease-Activating Factor 1; D003574:Cytochrome c Group; D003838:Deoxyadenine Nucleotides; D011506:Proteins; D011994:Recombinant Proteins; D020169:Caspases; C026600:2'-deoxyadenosine triphosphate; D011188:Potassium