Acute stimulation by lutropin of mitochondrial protein synthesis in small luteal cells

A two-dimensional electrophoretic technique was used to study the effect of acute stimulation of bovine luteal cells with lutropin on protein synthesis. Cells were incubated for 30 min with [35S]methionine in the presence of stimulating levels of luteinizing hormone (lutropin), after which the proteins were analyzed by autoradiography. Lutropin or N6,2'-O-dibutyryl-adenosine 3',5'-phosphate (Bt2cAMP) induced the labelling of three proteins, referred to as proteins A, B and C. Protein A, had a molecular mass of 28 kDa and an isoelectric point (pI) of 6.7. Proteins B and C had a molecular mass of 27 kDa and pI of 6.2 and 6.4 respectively. After subcellular fractionation, the three proteins were found to be markedly concentrated in the only fraction enriched in an established mitochondrial marker. Moreover, protein A was one of the major mitochondrial newly synthesized proteins. Its appearance was observed after a 5-min incubation and was prevented by 100 microM cycloheximide. The acute accumulation of proteins A, B and C in mitochondria, the site of the rate-limiting step of steroidogenesis, suggest that they could be involved in the mechanism of stimulation by lutropin of progesterone synthesis.