Towards a molecular understanding of titin.
Titin is at present the largest known protein (M(r) 3000 kDa) and its expression is restricted to vertebrate striated muscle. Single molecules span from M- to Z-lines and therefore over 1 micron. We have isolated cDNAs encoding five distant titin A-band epitopes, extended their sequences and determined 30 kb (1000 kDa) of the primary structure of titin. Sequences near the M-line encode a kinase domain and are closely related to the C-terminus of twitchin from Caenorhabditis elegans. This suggests that the function of this region in the titin/twitchin family is conserved throughout the animal kingdom. All other A-band sequences consist of 100 amino acid (aa) repeats predicting immunoglobulin-C2 and fibronectin type III globular domains. These domains are arranged into highly ordered 11 domain super-repeat patterns likely to match the myosin helix repeat in the thick filament. Expressed titin fragments bind to the LMM part of myosin and C-protein. Binding strength increases with the number of domains involved, indicating a cumulative effect of multiple binding sites for myosin along the titin molecule. We conclude that A-band titin is likely to be involved in the ordered assembly of the vertebrate thick filament.
Labeit
S
S
European Molecular Biology Laboratory, Heidelberg, FRG.
Gautel
M
M
Lakey
A
A
Trinick
J
J
D000595:Amino Acid Sequence; D000818:Animals; D002107:Caenorhabditis / Q000378:metabolism; D029742:Caenorhabditis elegans Proteins; D002148:Calmodulin-Binding Proteins; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism; D003001:Cloning, Molecular; D064211:Connectin; D004247:DNA / Q000235:genetics; D005353:Fibronectins / Q000235:genetics; D015801:Helminth Proteins / Q000235:genetics / Q000378:metabolism; D007136:Immunoglobulins / Q000235:genetics; D008969:Molecular Sequence Data; D009124:Muscle Proteins / Q000235:genetics* / Q000378:metabolism; D009218:Myosins / Q000378:metabolism; D016366:Open Reading Frames; D011494:Protein Kinases; D012091:Repetitive Sequences, Nucleic Acid; D012689:Sequence Homology, Nucleic Acid; D013728:Terminator Regions, Genetic
D029742:Caenorhabditis elegans Proteins; D002148:Calmodulin-Binding Proteins; D002352:Carrier Proteins; D064211:Connectin; D005353:Fibronectins; D015801:Helminth Proteins; D007136:Immunoglobulins; D009124:Muscle Proteins; C489233:unc-22 protein, C elegans; D004247:DNA; D011494:Protein Kinases; D009218:Myosins