Kinetics of translation of gamma B crystallin and its circularly permutated variant in an in vitro cell-free system: possible relations to codon distribution and protein folding

Analysis of nascent gamma B-crystallin peptides accumulating during in vitro translation in a rabbit reticulocyte lysate cell-free system was carried out. As a consequence of the irregular distribution of rare codons along the polypeptide chain of gamma B-crystallin, translation of the two-domain protein is a non-uniform process characterized by specific pauses. One of the major delays occurs during the translation of the connecting peptide between the domains. Comparing the kinetics of translation of natural gamma B-crystallin and its circularly permutated variant (with the order of the N- and C-terminal domains exchanged) reveals that the natural N-terminal domain is translated faster than the C-terminal one. Since the N-terminal domain in natural gamma B-crystallin is known to be more stable and to fold faster than the C-terminal one [E.-M. Mayr et al. (1994) J. Mol. Biol. 235, 84-88], the present data suggest that the translation rates are optimized to tune the synthesis and folding of the nascent polypeptide chain. In this connection, the pause in the linker region between the domains provides a delay allowing the correct folding of the N-terminal domain and its subsequent assistance in the stabilization of the C-terminal one.