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.