Invertebrate connectin spans as much as 3.5 µm in the giant sarcomeres of crayfish claw muscle.

In crayfish claw closer muscle, the giant sarcomeres are 8.3 pm long at rest, four times longer than vertebrate striated muscle sarcomeres, and they are extensible up to 13 pm upon stretch. Invertebrate connectin (1-connectin) is an elastic protein which holds the A band at the center of the sarcomere. The entire sequence of crayfish 1-connectin was predicted from cDNA sequences of 53 424 bp (17 352 residues; 1960 kDa). Crayfish 1-connectin contains two novel 68- and 71-residue repeats, and also two PEYK domains and one kettin region. Kettin is a small iso- form of 1-connectin. Immunobtot tests using antibody to the 68-residue repeats revealed the presence of 1-connectin also in tong sarcomeres of insect leg muscle and barnacle ventral muscle. Immunofluorescence microscopy demonstrated that the two repeats, the tong spacer and the two PEYK domains contribute to sarcomere extension. These regions rich in charged amino acids, occupying 63% of the crayfish 1-connectin molecule, may allow a span of a 3.5 p.m distance as a new class of composite spring. [ABSTRACT FROM AUTHOR]