Few weeks after birth, a switch in peptide elongation factor 1As from EF-1alpha/EF1A-1 to S1/EF1A-2 occurs in brain neurons, heart and skeletal muscles of mammalians. In order to elucidate the reason behind this switch, I studied the expression of both homologous proteins during muscle differentiation and apoptosis and, documented the relation between peptide elongation factors and caspase-3 activation. I found that during in vitro muscle differentiation of L6 myoblasts, a switch in peptide elongation factors 1A occurs as physiologically observed in skeletal muscles. While EF-1alpha/EF1A-1 is expressed in replicating myoblasts, S1/EF1A-2 is solely found in differentiated myotubes where it replaces EF-1alpha/EFIA-1 as the major elongation factor. Similarly, upon serum deprivation-induced apoptosis, a reversion in peptide elongation factors 1A is observed: EF-1alpha/EF1A-1 replaces S1/EF1A-2 and becomes the major form of elongation factor 1A present in dying myotubes. This switch correlates in myotubes with the activation of caspase-3 protein, a cysteine protease involved in apoptosis. When L6 myotubes constitutively express S1/EF1A-2 as caused by adenoviral gene transfer, they become resistant to serum deprivation-induced apoptosis. In contrast, when L6 myotubes are transfected with EF-1alpha/EF1A-1 gene, they die more rapidly from serum deprivation-induced apoptosis than control cells. Transfection using anti-sense EF-1alpha/EF1A-1 gene protects myotubes from apoptotic cell death. Thus, both elongation factor 1As exert opposing effect on muscle survival: while EF-1alpha/EF1A-1 accelerates apoptotic cell death, S1/EF1A-2 protects muscles against apoptosis. I found that skeletal muscles are the only tissues where, despite the constitutive expression of caspase-3 mRNA, the protein can be absent. Furthermore, I found that while immediately after birth, caspase-3 protein is present in skeletal muscles, a few weeks afterwards, the protein cannot be detected by Western blotting. In skeletal muscle, this change correlates with the observed switch in peptide elongation factors from EF-1alpha/EF1A-1 to S1/EF1A-2 and suggests that caspase-3 is translationally regulated in skeletal muscles. The laboratory previously reported that while EF-1alpha/EF1A-1 protein reappears; S1/EF1A-2 protein becomes absent from regenerating muscles. However, once tissue regeneration is completed, the situation returns to normal as EF-1alpha/EF1A-1 disappears and S1/EF1A-2 reappears to become the only type 1A elongation factor expressed in muscle. In conclusion, I found that the developmental switch observed in peptide elongation factors from EF-1alpha/EF1A-1 to S1/EF1A-2 partly serves to protect muscle cells from apoptosis. Thus, I am the first to identify a noncanonical function for S1/EF1A-2. (Abstract shortened by UMI.)

Wang, Eugenia (Supervisor)