The Folding of the Bifunctional TRP3 Protein in Yeast is Influenced by a Translational Pause which Lies in a Region of Structural Divergence with Escherichia coli Indoleglycerol‐Phosphate Synthase

The yeast TRP3 gene encodes a bifunctional protein with anthranilate synthase II and indoleglycerol‐phosphate synthase activities. Replacing ten consecutive non‐preferred codons in the indoleglycerol‐phosphate synthase region of the TRP3 gene with synonymous preferred codons (to create the TRP3pr gene; translational pause replaced) causes a 1.5‐fold reduction in relative indoleglycerol‐phosphate synthase activity [Crombie, T., Swaffield, J. C. & Brown, A. J. P. (1992) J. Mol. Biol. 228, 7–12]. Here, we report that both the anthranilate synthase II and indoleglycerol‐phosphate synthase domains are affected to similar extents when the translational pause is removed. Also, structural modelling of the yeast indoleglycerol‐phosphate synthase domain against the X‐ray crystal structure of indoleglycerol‐phosphate synthase from Escherichia coli indicates that the translational pause lies in a region of structural divergence between similar structures. To probe the role of cytoplasmic heat‐shock protein 70 (Hsp70) chaperones in Trp3 protein folding, anthranilate synthase and indoleglycerol‐phosphate synthase activities were measured in ssa and ssb mutants. Neither indoleglycerol‐phosphate synthase nor anthranilate synthase were affected significantly in the ssb mutant. However, depletion of Hsp70 proteins encoded by the SSA genes led to decreased anthranilate synthase and indoleglycerol‐phosphate synthase activities from the TRP3 gene, suggesting that both domains depend to some extent upon the SSA chaperone family. The data are consistent with roles for both the translational pause and Ssa chaperones in Trp3 protein folding in vivo.