The balance between two isoforms of the Drosophila RNA-binding protein howcontrols tendon cell differentiation

In Drosophila, a tendon cell is selected from a group of equipotent precurs ors following its interaction with a muscle cell. This interaction results in elevated levels of the transcription factor Stripe in the future tendon cells. Here we show that the balance between two distinct forms of the RNA- binding protein How maintains low levels of Stripe at the precursor stage a nd high levels in the mature tendon. The long, nuclear-specific protein How (L) downregulates Stripe protein levels at the precursor stage by binding s tripe mRNA and inhibiting its nuclear export. This inhibition is likely to be counteracted by the short How(S) protein, present in both nucleus and cy toplasm, which is upregulated in the muscle-bound tendon cell following EGF receptor activation.