Structural and functional studies of titin's fn3 modules reveal conserved surface patterns and binding to myosin S1--a possible role in the Frank-Starling mechanism of the heart

J Mol Biol. 2001 Oct 19;313(2):431-47. doi: 10.1006/jmbi.2001.5017.


The A-band part of titin, a striated-muscle specific protein spanning from the Z-line to the M-line, mainly consists of a well-ordered super-repeat array of immunoglobulin-like and fibronectin-type III (fn3)-like domains. Since it has been suspected that the fn3 domains might represent titin's binding sites to myosin, we have developed structural models for all of titin's 132 fn3-like domains. A subset of eight experimentally determined fn3 structures from a range of proteins, including titin itself, was used as homology templates. After grouping the models according to their position within the super-repeat segment of the central A-band titin region, we analyzed the models with respect to side-chain conservation. This showed that conserved residues form an extensive surface pattern predominantly at one side of the domains, whereas domains outside the central C-zone super-repeat region show generally less conserved surfaces. Since the conserved surface residues may function as protein-binding sites, we experimentally studied the binding properties of expressed multi-domain fn3 fragments. This revealed that fn3 fragments specifically bind to the sub-fragment 1 of myosin. We also measured the effect of fn3 fragments on the contractile properties of single cardiac myocytes. At sub-maximal Ca(2+) concentrations, fn3 fragments significantly enhance active tension. This effect is most pronounced at short sarcomere length, and as a result the length-dependence of Ca(2+) activation is reduced. A model of how titin's fn3-like domains may influence actomyosin interaction is proposed.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Sequence
  • Animals
  • Binding Sites
  • Calcium / pharmacology
  • Connectin
  • Conserved Sequence*
  • Fibronectins / chemistry*
  • Heart Ventricles / cytology
  • Heart Ventricles / drug effects
  • Humans
  • Immunoglobulins / chemistry
  • Mice
  • Mice, Inbred BALB C
  • Models, Molecular
  • Molecular Sequence Data
  • Muscle Proteins / chemistry*
  • Muscle Proteins / metabolism*
  • Muscle Proteins / pharmacology
  • Myocardial Contraction* / drug effects
  • Myocardium / cytology
  • Myocardium / metabolism*
  • Myosin Subfragments / metabolism*
  • Peptide Fragments / chemistry
  • Peptide Fragments / metabolism
  • Peptide Fragments / pharmacology
  • Protein Binding
  • Protein Kinases / chemistry*
  • Protein Kinases / metabolism*
  • Protein Kinases / pharmacology
  • Protein Structure, Tertiary
  • Rabbits
  • Sarcomeres / drug effects
  • Sarcomeres / metabolism
  • Sequence Alignment
  • Solvents / metabolism
  • Structure-Activity Relationship
  • Ventricular Function


  • Connectin
  • Fibronectins
  • Immunoglobulins
  • Muscle Proteins
  • Myosin Subfragments
  • Peptide Fragments
  • Solvents
  • TTN protein, human
  • Protein Kinases
  • Calcium