MICROTUBULE-ASSOCIATED PROTEIN-1 LIGHT-CHAIN-3 IS A FIBRONECTIN MESSENGER-RNA-BINDING PROTEIN LINKED TO MESSENGER-RNA TRANSLATION IN LAMB VASCULAR SMOOTH-MUSCLE CELLS

Intimal cushions form in the fetal ductus arteriosus by fibronectin-de pendent smooth muscle cell migration which is associated with greater efficiency of fibronectin mRNA translation, We investigated whether th e AU-rich element (ARE), UUAUUUAU, in the 3'-untranslated region (3'UT R) of fibronectin mRNA is involved in this mechanism by transfecting s mooth muscle cells with plasmids containing the chloramphenicol acetyl transferase coding region with its 3'UTR replaced by fibronectin 3'UTR bearing intact or mutated ARE, More efficient translation of fusion m RNA with intact versus mutated ARE was observed, This effect was ampli fied in ductus (10.9-fold) compared with nonmigratory, lower fibronect in-producing aorta cells (6.5-fold), Ductus cells transfected with wil d-type but not ARE-mutated plasmid reverted to the stellate phenotype of aorta cells associated with reduced fibronectin production, This su ggested that plasmid ARE sequesters RNA-binding factors, thereby reduc ing endogenous fibronectin mRNA translation, We next purified a 15-kD fibronectin ARE-dependent RNA-binding protein and identified it as mic rotubule-associated protein 1 light chain 3 (LC3), LC3 is present in g reater amounts in ductus compared with aorta cells, and overexpression of LC3 in aortic cells by transfection enhances fibronectin mRNA tran slation to levels observed in ductus cells.