Titin and nebulin: protein rulers in muscle?

Titin and nebulin are giant muscle proteins, both of which are approximately 1 micron long and are composed of many repeating domains. Titin domains resemble type III fibronectin and C-2 immunoglobulins. Both proteins are likely to be involved in specifying and stabilizing the highly ordered structure of muscle, probably by acting as 'protein rulers' to regulate the assembly of myosin and actin filaments precisely.