Titin PEVK segment: charge-driven elasticity of the open and flexible polyampholyte.
The giant protein titin spans half of the sarcomere length and anchors the myosin thick filament to the Z-line of skeletal and cardiac muscles. The passive elasticity of muscle at a physiological range of stretch arises primarily from the extension of the PEVK segment, which is a polyampholyte with dense and alternating-charged clusters. Force spectroscopy studies of a 51 kDa fragment of the human fetal titin PEVK domain (TP1) revealed that when charge interactions were reduced by raising the ionic strength from 35 to 560 mM, its mean persistence length increased from 0.30 +/- 0.04 nm to 0.60 +/- 0.07 nm. In contrast, when the secondary structure of TP1 was altered drastically by the presence of 40 and 80% (v/v) of trifluoroethanol, its force-extension behavior showed no significant shift in the mean persistence length of approximately approximately 0.18 +/- 0.03 nm at the ionic strength of 15 mM. Additionally, the mean persistence length also increased from 0.29 to 0.41 nm with increasing calcium concentration from pCa 5-8 to pCa 3-4. We propose that PEVK is not a simple entropic spring as is commonly assumed, but a highly evolved, gel-like enthalpic spring with its elasticity dominated by the sequence-specific charge interactions. A single polyampholyte chain may be fine-tuned to generate a broad range of molecular elasticity by varying charge pairing schemes and chain configurations.
Forbes
Jeffrey G
JG
Muscle Proteomics and Nanotechnology Section, Laboratory of Muscle Biology, NIAMS, NIH, DHHS, Bethesda, MD 20892-8024, USA.
Jin
Albert J
AJ
Ma
Kan
K
Gutierrez-Cruz
Gustavo
G
Tsai
Wanxia L
WL
Wang
Kuan
K
D000595:Amino Acid Sequence; D001696:Biomechanical Phenomena; D002021:Buffers; D002118:Calcium / Q000737:chemistry; D002942:Circular Dichroism; D064211:Connectin; D004548:Elasticity; D005091:Exons / Q000235:genetics; D006801:Humans; D007526:Isoelectric Point; D018625:Microscopy, Atomic Force; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009124:Muscle Proteins / Q000737:chemistry* / Q000235:genetics; D009994:Osmolar Concentration; D010446:Peptide Fragments / Q000737:chemistry / Q000235:genetics; D011487:Protein Conformation; D020033:Protein Isoforms / Q000737:chemistry / Q000235:genetics; D011494:Protein Kinases / Q000737:chemistry* / Q000235:genetics; D017433:Protein Structure, Secondary; D017386:Sequence Homology, Amino Acid; D013696:Temperature; D014270:Trifluoroethanol / Q000737:chemistry
D002021:Buffers; D064211:Connectin; D009124:Muscle Proteins; D010446:Peptide Fragments; D020033:Protein Isoforms; C577285:TTN protein, human; D014270:Trifluoroethanol; D011494:Protein Kinases; D002118:Calcium