Hydrophobic Mismatch between Proteins and Lipids in Membranes

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Hydrophobic Mismatch between Proteins and Lipids
in Membranes

• Susanne Pfeifer tiffy_at_tiffy.it
• 08.07.2004
• Seminar Theoretical Analysis of Protein-Protein
  InteractionsUniversität des SaarlandesChair of
  Prof. Dr. Volkhard Helms

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Agenda

• Introduction
• Possible adaptations to mismatch
• Consequences of mismatch for
• Proteins and peptides
• Lipid structure and organization
• Effects of mismatch in biomembranes

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BasicsIntroduction
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Basics Introduction
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BasicsIntroduction

• Length of lipid-exposed hydrophobic segments is
  equal to the hydrophobicbilayer thickness
• Proteins that are encountered in one membrane can
  have different lengths of their hydrophobic
  parts
• Membrane proteins with the same length can be
  encountered in bilayers of different thickness

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BasicsQuestions

1. How do membranes deal with a mismatch between
   the hydrophobic part of a transmembrane protein
   and the bilayer thickness?
2. How important is the extent ofhydrophobic
   matching for membrane structure and function?
3. Could mismatch play a functional role?

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BasicsPossible adaptations to mismatch

• Positive mismatch
• The protein might oligomerize or aggregate in
  the membrane to minimize the exposed hydrophobic
  area
• Transmembrane helices could tilt to reduce their
  effective hydrophobic length
• Transmembrane helices could adopt another
  conformation

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Basics Possible adaptations to mismatch

• Negative mismatch
• Results in protein aggregation or changes in
  backbone conformation or side chain orientation
• Too short peptides might not incorporate and
  adopt asurface localization
• Lipids decrease the bilayer thickness by
  disordering their acyl chains

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Basics Implications for membranes

• Effects on
• protein conformation
• protein orientation
• helical tilt
• aggregational behavior
• can affect
• protein activity
• membrane insertion
• protein assembly

• Effects on
• lipid structure
• lipid organization
• have implications for
• processes that are sensitive to lipid packing
• Processes that require the local and transient
  formation of non-lamellar structures

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Basics Consequences of mismatch

• Consequences for properties of proteins
• Protein activity and stability
• Protein aggregation
• Tilt
• Localization at membrane surface
• Protein/peptide backbone conformation

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Basics - Consequences of mismatch Protein
activity stability

• The extent of hydrophobic matching is important
  for determining the functional activity of
  proteins
• There are a number of proteins that do not show
  a clear optimum bilayer thickness for activity,
  but they require a minimal chain length
• many other factors may be involved in determining
  the functional activity of membrane
  proteins(e.g. lipid packing, fluidity, surface
  charge density, intrinsic curvature, lateral
  pressure profile, )
• ?Protein activity may be related to protein
  stability, which also can be affected by mismatch

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Basics - Consequences of mismatch Protein
aggregation

• Response to hydrophobic mismatch
• Occurred only with a rather large mismatch
• 4 Å thicker or
• 10 Å thinner
• than the estimated hydrophobic length of the
  proteinare allowed without induction of
  significant aggregation
• Proteins with long hydrophobic stretchtilt in
  the membrane
• Reduction of their effective length
• Comparison is difficult, because the lipids
  differ not only in acyl chain length, but also in
  other properties

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Basics - Consequences of mismatch Tilt

• Occurs if the hydrophobic part of a protein is
  too long to span the membrane
• Important for the functional and transport
  activity of membrane proteins
• An increase in helix tilt occurs at increasing
  protein content
• decrease in lipid order
• decrease in bilayer thickness
• Accompanied by a bend to reduce unfavorable
  effects on lipid packing

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Basics - Consequences of mismatch Tilt

• Change in helix tilt change in protein
  activity

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Basics - Consequences of mismatch Tilt

• Special cases
• In large proteinschanges in helical tilt have
  only little effect on lipid packing
• Single transmembrane helix a tilt would cause a
  strain on the surrounding lipids to accommodate
  the helix in the bilayer
• large degree of tilting is less favorable

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Basics - Consequences of mismatch Localization
at membrane surface

• Relatively small hydrophobic peptides may not be
  able to integrate into the membrane
• orientation at the membrane surface
• Peptide aggregation outside the bilayer
• Amino acid composition is important
• (in determining the consequences of hydrophobic
  mismatch)
• The extent of membrane insertion for amphipathic
  pore-forming peptides is mismatch dependent

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Basics - Consequences of mismatch Localization
at membrane surface

• Surface-absorbed peptides insert their
  hydrophobic side chains between the acyl chains
  near the membrane surface
• membrane-thinning effect
• dependent on the peptide/lipid ratio
• Important for studies
• on the mismatch dependence of insertion for
• such proteins
• insertion of hydrophobic peptides with an
  equilibrium between a transmembrane orientation
  and a surface localization

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Basics - Consequences of mismatch Backbone
conformation

• Helix length fluctuates due to local
• variations in backbone structure
• Sensitivity of the backbone conformation for
  environmental changes depends on amino acid
  composition
• Peptides with a hydrophobic stretch of
  alternating leucine and alanine are more
  sensitive than peptides with a polyleucine
  sequence

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BasicsConsequences of mismatch

• Consequences for lipid structure and
  organization
• Lipid chain order
• Phase transition temperature
• Preferential interactions andmicrodomain
  formation

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Basics - Consequences of mismatch Phase
transition temperature

• Melting transition temperature of lipid bilayers
  is strongly affected
• Proteins with long hydrophobic segments
• stabilize the thicker gel phase
• Short proteins stabilize the fluid phase

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Basics - Consequences of mismatch Microdomains

• In fluid bilayers consisting of lipids with
  different lengths, hydrophobic mismatch may
  induce preferential protein-lipid interactions ?
  formation of microdomains
• Systems consisting of two lipid species with
  different acyl chain lengths and one
  proteinhydrophobic mismatch induces
  preferential protein-lipid interactions
• (depending on hydrophobic length, differences in
  hydrophobic length)

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Basics Effects in biomembranes

• Protein sorting
• Membrane protein insertion and topology
• Regulation

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Basics - Effects in biomembranes Protein sorting

• Eukaryotic cell
• Level of cholesterol increases from the
  endoplasmatic reticulum via the Golgi to the
  plasma membrane
• (suggesting a concomitant increase in membrane
  thickness)
• Protein sorting in Golgi is based on this length
  difference
• Increasing the hydrophobic length of proteins
  that normally reside in the Golgi
• they can reroute the proteins to the plasma
  membrane (or vice versa)

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Basics - Effects in biomembranes Protein sorting

• Preferential protein-lipid interactions are
  consequences of hydrophobic mismatch
• results in domain formation and protein sorting

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Basics - Effects in biomembranes Membrane
protein insertion

• Signal sequences
• short hydrophobic length (7-15 amino acids)
• high tendency to form alpha-helical structures
• (with insufficient length to span a membrane)
• Length of signal sequences and mismatch are
  important for their functional activity
• A mismatch could lead to a local destabilization
  in a bilayer
• helps the translocation or
• promotes preferential interactions with other
  short helices of proteins in the translocation
  machinery

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Basics - Effects in biomembranes Membrane
protein insertion

• Signal anchors
• length closer to the hydrophobic thickness of
  the membrane (19-27 amino acids)
• ? influences the topology of proteins
• Stop transfer sequences
• ? hydrophobicity is more important than length

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Basics - Effects in biomembranes Membrane
thickness regulation

• A large variation in membrane thickness can be
  tolerated
• Variations of acyl chain length lead to changes
  in lipid composition
• important for surface charge density
• serves as tool to regulate local bilayer
  thickness
• ?prevention of unwanted consequences of
  hydrophobic mismatch in biological membranes

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BasicsResults

• Hydrophobic mismatch
• affects protein and lipid organisation
• affects conformation and thermodynamic
  properties of the membranes
• plays a role in protein sorting in vivo
• is required for specific functional properties
  of membranes
• depends on individual properties

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Chain Packing

• Calculation of all possible lipid conformations
• Probability of chain conformations relative to
  their distances
• Free interaction energy between two inclusions
• Detailed molecular-level information on chain
  conformational properties
• Problems
• Computationally expansive
• Full minimization of membrane shape is difficult

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Directors Model

• Theory-based model of elastic deformations is
  used to describe free energy differences
  associated with membrane perturbation due to
  protein-bilayer interactions
• (Huang, 1986 Helfrich and Jacobsson, 1990
  Nielsen et. al. 1998)
• All parameters were used beforein previous
  studies
• Thin, solvent-free lipid bilayer
• With an embedded inclusion similar to a
  gramicidin channel

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Directors Model - TheoryThe Model
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Directors Model - TheoryThe Model
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Directors Model - TheoryApproximation of changes

• Elastic modes for approximation of changes in
  lipid packing
• Compression-Expansion (CE)(due to changes in
  bilayer thickness)
• Splay-Distortion (SD)(due to variation in
  director among adjacent mol.)
• Surface-Tension (ST)(due to changes in bilayer
  surface area)

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Directors Model - TheoryTotal deformation free
energy

• compression-expansion

• surface tension

• splay-distortion

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Directors Model - ResultsChoice of boundary
conditions

• The bilayer deformation energy varies as a
  function of
• mechanical moduli
• boundary conditions
• ProblemEnergetic costs for packing the lipid
  molecules which are adjacent to the inclusion are
  not considered!

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Directors Model - ResultsChoice of boundary
conditions
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Directors Model - ResultsBilayer deformation
profile

• The shape of the deformation varies as a
  function of the elastic moduli
• Depending on the value of s, may the bilayer
  deformation profile be nonmonotonic
• Energy minimization requirement may causea
  compression adjacent to the inclusion and an
  expansion further away from the bilayer/inclusion
  boundery
• Packing Problemhydrophobic core volume per unit
  bilayer surface will deviate from its equilibrium
  value

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Directors Model - ResultsBilayer deformation
profile
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Directors Model - ResultsBilayer deformation
profile
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Directors Model - ResultsBilayer deformation
profile
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Directors Model - ResultsRadial decomposition of
free energy

• Depending on the choice of boundary conditions
  ?GCE can be less, equal or larger
• than ?GSD
• The relative contributions of these major
  components to ?Gdef vary in dependence of
• s (contact slope)
• ? (length scale)

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Directors Model - ResultsRadial decomposition of
free energy
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Directors Model - ResultsRadial decomposition of
free energy
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Directors Model - DiscussionComparison

• The results of the presented model confirm and
  extend the findings of Huang (1986) and Helfrich
  and Jakobsson (1990)
• Better results for s0
• Failures with ssmin could arise because the
  parameters that are used may be inappropriateor
  additional contributions to ?Gdef which are
  neglected
• Today there is insufficient information to choose
  the appropriate boundary conditions

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Directors Model - DiscussionBiological
implications
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AppendixReferences

• Hydrophobic mismatch between proteins
• and lipids in membranes (1998, Killian)
• Energetics of Inclusion-Induced Bilayer
  Deformations (1998, Nielson et al)
• A Molecular Model for Lipid-Protein Interactions
  in Membranes The Role of Hydrophobic Mismatch
  (1993, Deborah et al)
• Synthetic peptides as models for intrinsic
  membrane proteins (2003, Killian)

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