Upstream open reading frames regulate translation of Mona/Gads adapter mRNA in the megakaryocytic lineage

Mona, also called Gads, is a molecular adapter that plays a key role in T-cell and platelet signalling by linking the adaptors Src homology 2 domain-containing leukocyte phosphoprotein of 76 kDa (Slp-76) and linker for activation of T cells (LAT) upon T-cell receptor and collagen receptor activation. Platelets express a specific form of Mona mRNA, called 1B, which is transcribed from a megakaryocyte-specific promoter. Mona 1B mRNA differ from 1A transcripts found in T cells and some myeloid cells only by the 5'UTR. We report here that 1B mRNA expressing cells do not express detectable amounts of Mona protein, in contrast to 1A expressing cells, and we show that 1B 5'UTR contains upstream open reading frames (uORFs). Mutating the corresponding uAUG restored efficient Mona translation, or that of an unrelated ORF. This suggested that Mona protein expression in 1B mRNA expressing cells is tightly controlled at the translational level. Accordingly, Mona protein was not detected in resting platelets. Strikingly, platelet activation by thrombin resulted in the rapid induction of Mona protein expression, suggesting that translation inhibition of 1B mRNA may be relieved in activated platelets.