Properties of Chicken Lens MIP Channels Reconstituted into Planar Lipid Bilayers |
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Authors: | E Modesto P D Lampe M C Ribeiro D C Spray A C Campos de Carvalho |
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Institution: | (1) Instituto de Biofisica Carlos Chagas Filho, Universidade Federal do Rio de Janeiro, Rio de Janeiro, RJ, Brazil, BR;(2) Fred Hutchinson Cancer Research Center, Seattle, Washington, US |
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Abstract: | Membrane fractions highly enriched in chicken lens MIP (MIP28) were found to form ion channels when incorporated into planar
lipid bilayers. The channels displayed prominent unitary conductances of about 60 and 290 pS in symmetric 150 mm KCl solution and were slightly anion selective. For both depolarizing and hyperpolarizing voltages, voltage sensitivity of
the MIP28-induced conductance could be fit by a Boltzmann relation, symmetric around zero mV, with V
0
= 18.5 mV, n= 4.5 and g
min/g
max= 0.17. Channel properties were not appreciably altered by pH in the range of 5.8 to 7, although channel incorporation was
observed to occur more frequently at lower pH values. Calcium, at millimolar concentrations, decreased the channel mean open
time. Partial proteolysis of MIP28 to yield MIP21 did not appreciably affect single-channel conductance or voltage sensitivity
of the reconstituted channels. MIP28 was not phosphorylated by cAMP dependent protein kinase (PKA). Although unitary conductance
and selectivity of the chicken MIP channel are similar to those reported for the bovine MIP (MIP26), the voltage sensitivity
of MIP28 was higher than that of the bovine homologue, and voltage sensitivity of MIP28 was not modulated by treatments previously
shown to affect MIP26 voltage gating (partial proteolysis and protein phosphorylation by PKA: (Ehring et al., 1990). The existence
of such strikingly different functional properties in highly homologous channel isoforms may provide a useful system for exploration
of the structure-function relations of MIP channels.
Received: 27 March 1996/Revised: 5 August 1996 |
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Keywords: | : Single channels — Voltage gating — Phosphorylation — Proteolysis — MIP |
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