Crustacean frequenins: molecular cloning and differential localization at neuromuscular junctions

Crustacean muscles are innervated by phasic and tonic motor neurons that display differential physiology and have morphologically distinct synaptic terminals. Phasic motor neurons release much more transmitter per impulse and have filiform terminals, whereas tonic motor neurons release less transmitter and have larger terminals with prominent varicosities. Using an antibody raised against Drosophila frequenin (frq), a calcium-binding protein that enhances transmitter release in Drosophila synaptic terminals, we found that frq-like immunoreactivity is prominent in many of the phasic, but not tonic nerve endings of crayfish motor neurons. In contrast, synapsin- and dynamin-like immunoreactivities are strongly expressed in both types of terminal. The immunocytochemical findings strongly suggested the presence of an frq-like molecule in crayfish, and its differential expression indicated a possible modulatory role in transmitter release. Therefore, we cloned the cDNA sequences for the crayfish and lobster homologues of Drosophila frq. Crustacean frequenins are very similar in sequence to their Drosophila counterpart, and calcium-binding regions (EF hands) are conserved. The widespread occurrence of frq-like molecules and their differential localization in crayfish motor neurons indicate a significant role in physiology or development of these neurons.