MAP1 structural organization in Drosophila: in vivo analysis of FUTSCH reveals heavy- and light-chain subunits generated by proteolytic processing at a conserved cleavage site

The MAP1 (microtubule-associated protein 1) family is a class of microtubule-binding proteins represented by mammalian MAP1A, MAP1B and the recently identified MAP1S. MAP1A and MAP1B are expressed in the nervous system and thought to mediate interactions of the microtubule-based cytoskeleton in neural development and function. The characteristic structural organization of mammalian MAP1s, which are composed of heavy- and light-chain subunits, requires proteolytic cleavage of a precursor polypeptide encoded by the corresponding map1 gene. MAP1 function in Drosophila appears to be fulfilled by a single gene, futsch. Although the futsch gene product is known to share several important functional properties with mammalian MAP1s, whether it adopts the same basic structural organization has not been addressed. Here, we report the identification of a Drosophila MAP1 light chain, LC_f, produced by proteolytic cleavage of a futsch-encoded precursor polypeptide, and confirm co-localization and co-assembly of the heavy chain and LC_f cleavage products. Furthermore, the in vivo properties of MAP1 proteins were further defined through precise MS identification of a conserved proteolytic cleavage site within the futsch-encoded MAP1 precursor and demonstration of light-chain diversity represented by multiple LC_f variants. Taken together, these findings establish conservation of proteolytic processing and structural organization among mammalian and Drosophila MAP1 proteins and are expected to enhance genetic analysis of conserved MAP1 functions within the neuronal cytoskeleton.