Lutropin increases phosphorylation of a 33 000-dalton ribosomal protein in rat tumour Leydig cells.

Addition of lutropin (luteinizing hormone, 'LH') and 3-isobutyl-1-methylxanthine to tumour Leydig cells stimulated phosphorylation of five proteins, of 17 000, 22 000, 24 000, 33 000 and 57 000 Da. Phosphorylation of these proteins coincided with increased pregnenolone production. Phosphorylation of a 33 000-Da protein was lutropin-dependent in Leydig cells isolated from a Leydig-cell tumour, from immature testes or from mature testes. In tumour Leydig cells this protein was present in the small ribosomal subunit. Incubation of tumour Leydig cells with either cycloheximide or puromycin inhibited both basal and lutropin-dependent pregnenolone production, by approx. 90% and 98% respectively. In contrast, basal pregnenolone production in Leydig cells from immature and mature testes was insensitive to cycloheximide or puromycin. Cycloheximide or puromycin increased phosphorylation of the 33 000-Da phosphoprotein by approx. 130% and 80% respectively (effect of lutropin/3-isobutyl-1-methylxanthine on phosphorylation: 100%). The molecular mass, the subcellular localization and the sensitivity to phosphorylation in the presence of inhibitors of protein synthesis indicate that the 33 000-Da protein could be similar to ribosomal protein S6.