Selectivity of connexin-specific gap junctions does not correlate with channel conductance.
Connexins form a variety of gap junction channels that vary in their developmental and tissue-specific levels of expression, modulation of gating by transjunctional voltage and posttranslational modification, and unitary channel conductance (gamma j). Despite a 10-fold variation in gamma j, whether connexin-specific channels possess distinct ionic and molecular permeabilities is presently unknown. A major assumption of the conventional model for a gap junction channel pore is that gamma j is determined primarily by pore diameter. Hence, molecular size permeability limits should increase and ionic selectivity should decrease with increasing channel gamma j (and pore diameter). Equimolar ion substitution of 120 mmol/L KCl for potassium glutamate was used to determine the unitary conductance ratios for rat connexin40 and connexin43, chicken connexin43 and connexin45, and human connexin37 channels functionally expressed in communication-deficient mouse neuroblastoma (N2A) cells. Comparison of experimental and predicted conductance ratios based on the aqueous mobilities of all ions according to the Goldman-Hodgkin-Katz current equation was used to determine relative anion-to-cation permeability ratios. Direct correlation of junctional conductance with dye transfer of two fluorescein-derivatives (2 mmol/L 6-carboxyfluorescein or 2',7'-dichlorofluorescein) was also performed. Both approaches revealed a range of selectivities and permeabilities for all five different connexins that was independent of channel conductance. These results are not consistent with the conventional simple aqueous pore model of a gap junction channel and suggest a new model for connexin channel conductance and permselectivity based on electrostatic interactions. Divergent conductance and permeability properties are features of other classes of ion channels (eg, Na+ and K+ channels), implying similar mechanisms for selectivity.
Veenstra
R D
RD
Department of Pharmacology, State University of New York Health Science Center at Syracuse 13210, USA.
Wang
H Z
HZ
Beblo
D A
DA
Chilton
M G
MG
Harris
A L
AL
Beyer
E C
EC
Brink
P R
PR
D000818:Animals; D015152:Blotting, Northern; D002478:Cells, Cultured; D002645:Chickens; D017630:Connexins / Q000235:genetics / Q000378:metabolism*; D004058:Diffusion; D004553:Electric Conductivity; D004594:Electrophysiology; D005452:Fluoresceins; D005456:Fluorescent Dyes; D017629:Gap Junctions / Q000378:metabolism* / Q000502:physiology; D005822:Genetic Vectors; D006801:Humans; D007473:Ion Channels / Q000235:genetics / Q000378:metabolism* / Q000502:physiology; D008081:Liposomes; D051379:Mice; D008953:Models, Anatomic; D008954:Models, Biological; D008970:Molecular Weight; D009447:Neuroblastoma; D018408:Patch-Clamp Techniques; D010714:Phosphatidylethanolamines; D010957:Plasmids / Q000235:genetics; D051381:Rats; D015290:Second Messenger Systems; D013499:Surface Properties; D014162:Transfection
D017630:Connexins; D005452:Fluoresceins; D005456:Fluorescent Dyes; D007473:Ion Channels; D008081:Liposomes; D010714:Phosphatidylethanolamines; C024098:6-carboxyfluorescein; C037631:2',7'-dichlorofluorescein; C020888:1,2-dielaidoylphosphatidylethanolamine