Titin is the largest protein known, and is essential for organising muscle 
sarcomeres. It has many domains with a variety of functions, and stretches from 
the Z-line to the M-line in the muscle sarcomere. Close to the M-line, titin 
contains a kinase domain, which is known to phosphorylate the Z-line protein 
telethonin in developing muscle (Mayans, O., van der Ven, P. F., Wilm, M., Mues, 
A., Young, P., Furst, D. O., Wilmanns, M. and Gautel, M. (1998) Nature 395, 
863-869). This phosphorylation is thought to be important for initiating or 
regulating myofibrillogenesis. We used a gene-targeting approach in cultured 
myoblasts to truncate the titin gene so that the kinase domain and other domains 
downstream of the kinase were not expressed. We recovered cells in which one 
allele was targeted. We found that these cells expressed both the full-length 
and a truncated titin that was approximately 0.2 MDa smaller than the 
corresponding band from wild-type cells. Myofibrillogenesis in these cells was 
impaired, in that the myotubes were shorter, and the organisation of the muscle 
sarcomeres, M- and Z-lines was poorer than in wild-type cells. There was also an 
overall reduction in levels of titin and skeletal myosin expression. These 
results suggest that the activity of the titin kinase domain and downstream 
sequence are important in organising myofibrils both at the M- and the Z-line 
early in myofibrillogenesis.