THE CENTRAL Z-DISK REGION OF TITIN IS ASSEMBLED FROM A NOVEL REPEAT IN VARIABLE COPY NUMBERS

The giant sarcomeric protein titin (also described as connectin) is co mposed mainly of immunoglobulin (Ig)like and fibronectin type III (fn3 )-like domains arranged consecutively, At both ends of the molecule, t hese domains are interrupted by sequence insertions, The amino terminu s of titin is localized in the Z-disk. a structure of great variabilit y in different muscle types. We have determined the ultrastructural po sition of sequences in this region of the molecule in skeletal and car diac muscle by immunoelectron microscopy using antibodies directed aga inst unique epitopes. Titin molecules entering the Z-disk from two hal f sarcomeres do not significantly overlap, showing that the amino term inus is at the centre of the Z-disk. A serine/proline rich site, which can be phosphorylated by kinases in developing muscle tissues, was id entified near the amino terminus of titin. Sequence analysis revealed the presence of a novel 45 residue repeat ('Z-repeats') in this region of the molecule, The number of titin Z-repeats varies due to differen tial splicing, We propose that this mechanism is a means of assembling Z-disks of variable thickness and mechanical strength.