Spatial and temporal requirement of Mlp60A isoforms during muscle development and function in Drosophila melanogaster

Many myofibrillar proteins undergo isoform switching in a spatio-temporal manner during muscle development. The biological significance of the variants of several of these myofibrillar proteins remains elusive. One such myofibrillar protein, the Muscle LIM Protein (MLP), is a vital component of the Z-discs. In this paper, we show that one of the Drosophila MLP encoding genes, Mlp60A, gives rise to two isoforms: a short (279 bp, 10 kDa) and a long (1461 bp, 54 kDa) one. The short isoform is expressed throughout development, but the long isoform is adult-specific, being the dominant of the two isoforms in the indirect flight muscles (IFMs). A concomitant, muscle-specific knockdown of both isoforms leads to late pupal lethality, with the surviving flies being majorly flight defective. Mlp60A null flies show developmental lethality, and muscle defects in the individuals surviving till the third instar larval stage. This lethality could be rescued partially by muscle-specific overexpression of the short isoform. Almost 90% of the long isoform-specific P-element insertion mutant flies show a compromised flight ability and have reduced sarcomere length. Hence, our data shows that the two Mlp60A isoforms are functionally specialized, to ensuring normal embryonic muscle development and adult flight muscle function.