Hydrophobic Mismatch between Proteins and Lipids in Membranes Susanne Pfeifer [email protected]...
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Transcript of Hydrophobic Mismatch between Proteins and Lipids in Membranes Susanne Pfeifer [email protected]...
Hydrophobic Mismatch between Proteins and Lipids in Membranes
Susanne Pfeifer [email protected]
Seminar Theoretical Analysis of Protein-Protein Interactions
Universität des SaarlandesChair of Prof. Dr. Volkhard Helms
2
Agenda
• Introduction• Possible adaptations to mismatch• Consequences of mismatch for:
• Proteins and peptides• Lipid structure and organization
• Effects of mismatch in biomembranes
6
Basics
Introduction
• 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
7
Basics
Questions
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?
8
Basics
Possible 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
9
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
10
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
11
Basics
Consequences of mismatch
• Consequences for properties of proteins• Protein activity and stability• Protein aggregation• Tilt• Localization at membrane surface• Protein/peptide backbone conformation
12
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
13
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
14
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
16
Basics - Consequences of mismatch
Tilt
• Special cases:• In large proteins:
changes 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
17
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
18
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
19
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
20
Basics
Consequences of mismatch
• Consequences for lipid structure and organization• Lipid chain order• Phase transition temperature• Preferential interactions and
microdomain formation
21
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
22
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 protein:hydrophobic mismatch induces preferential protein-lipid interactions(depending on hydrophobic length, differences in hydrophobic length…)
23
Basics
Effects in biomembranes
• Protein sorting• Membrane protein insertion and
topology• Regulation
24
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)
25
Basics - Effects in biomembranes
Protein sorting
• Preferential protein-lipid interactions are consequences of hydrophobic mismatch results in domain formation and protein
sorting
26
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 orpromotes preferential interactions with other short
helices of proteins in the translocation machinery
27
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
28
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
thicknessprevention of unwanted consequences of hydrophobic mismatch in biological membranes
29
Basics
Results
• 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
30
Chain Packing
• Calculation of all possible lipid conformations• Probability of chain conformations
relative to their distances• Free interaction energy between
two inclusionsDetailed molecular-level information
on chain conformational properties
• Problems:• Computationally expansive• Full minimization of membrane
shape is difficult
31
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
34
Directors Model - Theory
Approximation 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)
35
Directors Model - Theory
Total deformation free energy
• compression-expansion
rdrdr
du
dr
ud
dr
du
rKu
d
KG
r
r cdef
a
0
2222
02
14
rdrud
KG
r
rCE
a
0
2
02
4
rdrdr
ud
dr
du
rKG
r
r cSD
0
221
rdrdr
duG
r
rST
0
2
• surface tension
• splay-distortion
36
Directors Model - Results
Choice of boundary conditions• The bilayer deformation energy varies
as a function of• mechanical moduli• boundary conditions
• Problem:Energetic costs for packing the lipid molecules which are adjacent to the inclusion are not considered!
38
Directors Model - Results
Bilayer 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 cause
a 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
39
Directors Model - Results
Bilayer deformation profile
rdrdr
du
dr
ud
dr
du
rKu
d
KG
r
r cdef
a
0
2222
02
14
40
Directors Model - Results
Bilayer deformation profile
rdrdr
du
dr
ud
dr
du
rKu
d
KG
r
r cdef
a
0
2222
02
14
41
Directors Model - Results
Bilayer deformation profile
rdrdr
du
dr
ud
dr
du
rKu
d
KG
r
r cdef
a
0
2222
02
14
42
Directors Model - Results
Radial decomposition of free energy• Depending on the choice of boundary
conditions GCE can be less, equal or largerthan GSD
• The relative contributions of these major components to Gdef vary in dependence of• s (contact slope) (length scale)
4
1204
1
a
c
K
dK
45
Directors Model - Discussion
Comparison
• The results of the presented model confirm and extend the findings of Huang (1986) and Helfrich and Jakobsson (1990)• Better results for s=0
• Failures with s=smin 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
47
Appendix
References
• 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)