Myomesin is an end line protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 that is part of the M line. It is a protein found in the M-band of muscle sarcomere

Sarcomere

A sarcomere is the basic unit of a muscle. Muscles are composed of tubular muscle cells . Muscle cells are composed of tubular myofibrils. Myofibrils are composed of repeating sections of sarcomeres, which appear under the microscope as dark and light bands...

s in association with M-protein. It is found in both slow and fast muscle fibers while M-protein is only found in fast fibers. These proteins are thought to be involved in anchoring the thick filaments of the sarcomere (myosin) to other filaments, namely titin

Titin

Titin , also known as connectin, is a protein that in humans is encoded by the TTN gene. Titin is a giant protein that functions as a molecular spring which is responsible for the passive elasticity of muscle. It is composed of 244 individually folded protein domains connected by unstructured...

, stabilizing and aligning the structure. It may function as a molecular spring that protects the sarcomere and keeps it stable during intense or sustained stretching. While its specific function is still not completely understood, it has been shown that it is closely associated with myosin as it is present in all skeletal and cardiac muscle from the earliest stages of muscle tissue formation.