COTRANSLATIONAL FOLDING OF GLOBIN

Globin synthesis in a wheat germ cell-free translation system was perf ormed in the presence of [H-3]hemin and [S-35]methionine to determine the minimal length of the nascent ribosome-bound globin chain capable of heme binding, Nascent polypeptides of predetermined size were synth esized on ribosomes by translation of truncated mRNA molecules. Analys is with the use of sucrose gradient centrifugation and puromycin react ion revealed that the ribosome-bound N-terminal alpha-globin fragments of 140, 100, and 86 amino acid residues are capable of an efficient h eme binding, whereas those of 75, 65, and 34 amino acid residues displ ay a significantly weaker, or just nonspecific, affinity to heme. This indicates that the ribosome-bound nascent chain of 86 amino acid resi dues has already acquired a spatial structure that allows its interact ion with the heme group or that heme attachment promotes the formation of the proper tertiary structure in the ribosome-bound nascent peptid e, In any case the cotranslational folding of globin is suggested.