Final answer:
The M line is the structure composed of protein filaments found in the center of the thick filaments.
Step-by-step explanation:
The structure composed of protein filaments and located in the center of the thick filaments is called the M line. The M line is a vertical line found in the middle of the H zone in a sarcomere, which is the basic unit of contraction in a muscle. It serves as an attachment site for accessory proteins that hold the thick filaments together.