Within the cilia and the flagella of eukaryotes is a microtubule-based cytoskeleton referred to as the axoneme. A cross-section of the axoneme reveals nine sets of microtubule doublets forming a ring inside the cilium or flagellum. (Motile cilia and flagella have an additional two singlet microtubules at the center apart from the ring of microtubule doublets, thus forming a " 9+2" configuration).
Connecting the microtubule doublet to the adjacent doublet is the interdoublet link. The role of interdoublet link is not yet fully understood but it may be providing support to the axoneme during an active doublets sliding.1
Nexin is the interdoublet link protein responsible for the maintenance of the nine-fold configuration in cilia and flagella. It was first isolated by R.E. Stephens. In his report in 1970, he named this protein nexin, from the Latin nexus, which means "a tie binding together members of a group". 2 He was able to isolate nexin from Strongylocentrotus drocbachiensis, Asterias sp., and Arbacia punctulata sperm flagella through SDS-PAGE. The protein, according to Stephens, represents about 2% of the total axonemal protein, and less than 1% of the total flagellar protein. Its molecular weight is 165,000 +/- 10%.2
1 Bourne, G. (1987). Cytology and cell physiology (4th ed.). San Diego: Academic Press.
2 Stephens, R.E. (1970). Isolation of nexin—the linkage protein responsible for maintenance of the nine-fold configuration of flagellar axonemes. Biol Bull 139: 438.