Crustacean frequenins: Molecular cloning and differential localization at neuromuscular junctions

Crustacean muscles are innervated by phasic and tonic motor neurons that di splay differential physiology and have morphologically distinct synaptic te rminals. Phasic motor neurons release much more transmitter per impulse and have filiform terminals, whereas tonic motor neurons release less transmit ter and have larger terminals with prominent varicosities, Using an antibod y raised against Drosophila frequenin (frq), a calcium-binding protein that enhances transmitter release in Drosophila synaptic terminals, we found th at frq-like immunoreactivity is prominent in many of the phasic, but not to nic nerve endings of crayfish motor neurons. In contrast, synapsin- and dyn amin-like immunoreactivities are strongly expressed in both types of termin al, The immunocytochemical findings strongly suggested the presence of an f rq-like molecule in crayfish, and its differential expression indicated a p ossible modulatory role in transmitter release. Therefore, we cloned the cD NA sequences for the crayfish and lobster homologues of Drosophila frq, Cru stacean frequenins are very similar in sequence to their Drosophila counter part, and calcium-binding regions (EF hands) are conserved, The widespread occurrence of frq-like molecules and their differential localization in cra yfish motor neurons indicate a significant role in physiology or developmen t of these neurons. (C) 1999 John Wiley & Sons, Inc.