Answer
The proteins, actin and titin, connect in the Z disc.
Work Step by Step
Actin, like myosin is a contractile protein. It is found in thin filaments where it binds to myosin heads.
Titin is a structural protein. It is a large molecule that connects a Z disc to an M line in each sarcomere; this stabilizes the thick filament. Titin confers elasticity and extensibility on myofibrils.