The function of the M-line protein, obscurin, in controlling the symmetry of the sarcomere inDrosophilaflight muscle

Abstract

There was an error published in J. Cell Sci. 125, 3367-3379.In the section 'Effect of reduced obscurin expression on other proteins in IFM', the second sentence was published omitting the term 'M-line'. The correct sentence should read as follows:In wild-type myofibrils, zormin was in both Z-disc and M-line (supplementary material Fig. S3A).On p.3373, left column, second paragraph, second line, the word 'line' was published twice.We apologise for these mistakes. Summary Obscurin (also known as Unc-89 in Drosophila) is a large modular protein in the M-line of Drosophila muscles. Drosophila obscurin is similar to the nematode protein UNC-89. Four isoforms are found in the muscles of adult flies: two in the indirect flight muscle (IFM) and two in other muscles. A fifth isoform is found in the larva. The larger IFM isoform has all the domains that were predicted from the gene sequence. Obscurin is in the M-line throughout development of the embryo, larva and pupa. Using P-element mutant flies and RNAi knockdown flies, we have investigated the effect of decreased obscurin expression on the structure of the sarcomere. Embryos, larvae and pupae developed normally. In the pupa, however, the IFM was affected. Although the Z-disc was normal, the H-zone was misaligned. Adults were unable to fly and the structure of the IFM was irregular: M-lines were missing and H-zones misplaced or absent. Isolated thick filaments were asymmetrical, with bare zones that were shifted away from the middle of the filaments. In the sarcomere, the length and polarity of thin filaments depends on the symmetry of adjacent thick filaments; shifted bare zones resulted in abnormally long or short thin filaments. We conclude that obscurin in the IFM is necessary for the development of a symmetrical sarcomere in Drosophila IFM.

Obscure functions: the location–function relationship of obscurins

“…In the mature fly, four isoforms (A, B, C, and D) are expressed in the thorax, while two of these isoforms (A and C) are also expressed in the indirect flight muscles (IFM). On the contrary, the larva expresses only obscurin isoform-E (Katzemich et al 2012(Katzemich et al , 2015. Unfortunately, their protein composition is not well annotated and lacks experimental confirmation; herein we provide the domain architecture for each predicted isoform (Fig 1).…”

“…More recently, the Drosophila homolog of obscurin has also been identified ( Fig. 1) (Katzemich et al 2012(Katzemich et al , 2015. Specifically, five obscurin isoforms are expressed in Drosophila in a developmental and tissue-specific manner.…”

“…Drosophila obscurin isoforms possess a common COOHterminal domain, which includes two FnIII domains an Ig domain and two pseudo-kinase domains. Unlike mammalian obscurin-A, there is no analogous ankyrin-binding domain in Drosophila obscurin or C. elegans UNC-89 (Katzemich et al 2012(Katzemich et al , 2015.…”

“…In addition, a zebrafish model depleted of obscurins resulted in myofibril disarray and impaired lateral alignment of adjacent myofibrils in both cardiac and skeletal muscle (Raeker et al 2006). In the fly, decreased expression of Drosophila obscurin in the IFM during pupal development negatively affects myosin assembly (Katzemich et al 2012), and knockdown of Drosophila obscurins in the adult IFM results in extensive disarray of thick filaments, sarcomeres lacking M-bands and flightless flies (Katzemich et al 2012). Also, through their pseudo-kinase domains Drosophila obscurins interact with Ball, a Ser/Thr kinase, and MASK, a protein with multiple ankyrin repeats (Katzemich et al 2015).…”

Binding partners of the kinase domains inDrosophilaobscurin and their effect on the structure of the flight muscle

“…Both Unc-89 (the obscurin homologue in Caenorhabditis elegans; note that this protein is also known as Unc-89 in Drosophila) and obscurin in Drosophila have SH3 and Rho-GEF signalling domains near the N-terminus and two kinase domains near the C-terminus (Benian et al, 1996;Katzemich et al, 2012;Small et al, 2004). In vertebrate obscurin, the signalling domains are near the C-terminus; the isoform obscurin A has an ankyrin-binding domain instead of the two C-terminal kinase domains in obscurin B.…”