Structure of the Drosophila projectin protein: isoforms and implication for projectin filament assembly

The protein composition of the various muscle types in Drosophila melanogaster has been studied quite thoroughly and the analysis has revealed many differences involving the usage of muscle specific isoforms of a given protein, as well as the presence of proteins restricted to one muscle type. Drosophila projectin, the giant protein component of the third filament is quite unusual as it not only shows specific isoforms in various muscle types, but these isoforms are located at different sarcomeric locations, I band in the IFM and A band in synchronous muscles. This may suggest distinct functions for the projectin protein in various muscles, as well as a different set of protein interactions for each projectin isoform. Projectin is encoded by a single gene and the isoforms were proposed to be the result of alternative splicing of a primary transcript. Here, we report the nearly complete sequence of Drosophila projectin, as well as the possible splicing patterns used to generate different isoforms. The overall domain organization in projectin is composed of repeated motifs I and II in a few specific patterns, similar to its Caenorhabditis homolog, twitchin. Sequence similarity between twitchin and projectin further suggests how some domains may possibly be important for protein interactions and/or functions. Alternative splicing operates at the COOH terminus, leading to a shorter projectin protein lacking some of the terminal motifs II and unique sequence. These isoforms are discussed in view of projectin differential size and localization.