Polar groups in membrane channels: Consequences of replacing alanines with serines in membrane-spanning gramicidin channels Academic Article uri icon

Overview

MeSH Major

  • Alanine
  • Gramicidin
  • Ion Channels
  • Serine

abstract

  • To explore the consequences of burying polar, hydrogen-bonding hydroxyl groups within the hydrocarbon core of lipid bilayer membranes, we examined the structural and functional effects of alanine-to-serine substitutions in bilayer-spanning gramicidin channels. A native Ala was replaced by Ser at position 3 or 5 in the gramicidin A (gA) sequence: formyl-VG(2)A(3)LA(5)VVVWLWLWLW-ethanolamide (d-residues underlined). In the head-to-head dimers that form the conducting, membrane-spanning gA channels, these sequence positions are located near the lipid bilayer center (and subunit interface). The sequence substitutions at positions 3 and 5 were tested within the context of having either Gly or d-Ala at position 2, because d-Ala(2) causes the channel lifetimes to increase 3-fold relative to Gly(2) [Mattice et al. (1995) Biochemistry 34, 6827]. Size-exclusion chromatograms and circular dichroism spectra show that the Ala --> Ser replacements are well tolerated and have little effect on channel structure. In planar bilayers, the Ser-substituted gramicidins form well-defined channels, with cation conductances that are approximately 60% of those of the reference channels. The Ser-substituted channels are structurally equivalent to native gramicidin channels, as demonstrated by the formation of heterodimeric channels between a Ser-containing subunit and a native gramicidin subunit. These hybrid channels exhibit rectification, attributable to asymmetric placement of the single Ser hydroxyl group with respect to the bilayer center. Compared to the corresponding Ala-containing reference channels, the polar Ser residues decrease the analogues' channel-forming potency by 3 orders of magnitude, indicating a substantial energetic penalty ( approximately 15 kJ/mol) for burying the polar Ser side chain in the bilayer hydrophobic core.

publication date

  • August 17, 2010

Research

keywords

  • Academic Article

Identity

Language

  • eng

PubMed Central ID

  • PMC2921912

Digital Object Identifier (DOI)

  • 10.1021/bi100857g

PubMed ID

  • 20695525

Additional Document Info

start page

  • 6856

end page

  • 65

volume

  • 49

number

  • 32