Q.Name the protein that forms the thick filament of a sarcomere and the protein that forms its thin filament.

[!TLDR]

Myosin forms the thick filament and actin forms the thin filament of a sarcomere.

[!ANSWER]

Myosin (thick filament); actin (thin filament).

The thick filament of a sarcomere is built of many molecules of the protein myosin, each with a rod-shaped tail and a globular, ATPase-active head. The thin filament is built mainly of the protein actin, whose individual G-actin monomers polymerise into the double-helical F-actin that forms the filament's backbone, with the regulatory proteins tropomyosin and troponin also associated with it.

[!ANSWER]

Myosin (thick filament); actin (thin filament).

Pair 'thick filament' with myosin and 'thin filament' with actin -- the two names correspond directly to the two filament types of a sarcomere.

Reversing the pairing (calling actin the thick filament or myosin the thin filament) -- myosin's larger molecule with a projecting head is what makes the thick filament thick.