Maitotoxin converts the plasmalemmal Ca(2+) pump into a Ca(2+)-permeable nonselective cation channel.
Maitotoxin (MTX) activates Ca(2+)-permeable nonselective cation channels and causes a dramatic increase in cytosolic free Ca(2+) concentration ([Ca(2+)](i)) in every cell examined to date, but the molecular identity of the channels involved remains unknown. A clue came from studies of a structurally related marine toxin called palytoxin (PTX). PTX binds to the plasmalemmal Na(+)-K(+)-ATPase (NKA) and converts the Na(+) pump into a nonselective cation channel. Given the high permeability of the MTX channel for Ca(2+), we considered the possibility that MTX may bind to the plasmalemmal Ca(2+)-ATPase (PMCA) pump, and like PTX, convert the pump into a channel. To test this hypothesis, the PMCA was overexpressed in Spodoptera frugiperda (Sf9) insect cells and in human embryonic kidneys (HEK) 293 cells. In both cell types, enhanced expression of the PMCA was associated with a significant increase in MTX-induced whole cell membrane currents. The effect of MTX on whole cell currents in both wild-type and PMCA overexpressing HEK cells was sensitive to pump ligands including Ca(2+) and ATP. MTX-induced currents were significantly reduced by knockdown of PMCA1 in HEK cells using small interfering RNA or in mouse embryonic fibroblasts from genetically modified mice with the PMCA1(+/-) PMCA4(-/-) genotype. Finally, PMCA catalytic activity (i.e., Ca(2+)-ATPase) in isolated membranes, or in purified PMCA preparations, was inhibited by MTX. Together, these results suggest that MTX binds to and converts the PMCA pump into a Ca(2+)-permeable nonselective cation channel.
Department of Physiology, Rammelkamp Center for Education and Research, Case Western Reserve University School of Medicine, Cleveland, Ohio, USA.
William G
WG
Sinkins
Mark
M
Estacion
Vikram
V
Prasad
Monu
M
Goel
Gary E
GE
Shull
Diana L
DL
Kunze
William P
WP
Schilling
D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D030801:Animals, Genetically Modified; D002118:Calcium / Q000378:metabolism*; D027682:Cation Transport Proteins / Q000187:drug effects* / Q000378:metabolism*; D002412:Cations / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism / Q000502:physiology; D002478:Cells, Cultured; D015536:Down-Regulation; D004553:Electric Conductivity; D005347:Fibroblasts / Q000378:metabolism; D006801:Humans; D007473:Ion Channels / Q000378:metabolism*; D007668:Kidney / Q000166:cytology / Q000187:drug effects / Q000196:embryology / Q000378:metabolism; D008387:Marine Toxins / Q000378:metabolism / Q000494:pharmacology*; D051379:Mice; D035964:Oxocins / Q000378:metabolism / Q000494:pharmacology*; D010539:Permeability; D053499:Plasma Membrane Calcium-Transporting ATPases / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D034741:RNA, Small Interfering / Q000494:pharmacology; D018411:Spodoptera; D015854:Up-Regulation; D000095702:Polyether Toxins
D000255:Adenosine Triphosphate; D002118:Calcium; D027682:Cation Transport Proteins; D002412:Cations; D007473:Ion Channels; D008387:Marine Toxins; D035964:Oxocins; D053499:Plasma Membrane Calcium-Transporting ATPases; D034741:RNA, Small Interfering; C020394:maitotoxin; D000095702:Polyether Toxins; C507517:ATP2B4 protein, human