Primary structure determinants of the pH- and temperature-dependent aggregation of thioredoxin

Thioredoxins are small proteins found in all living organisms. We have previously reported that Chlamydomonas reinhardtii thioredoxin h exhibited differences both in its absorption spectrum and its aggregation properties compared to thioredoxin m. In this paper, we demonstrate, by site-directed mutagenesis, that the particularity of the absorption spectrum is linked to the presence of an additional tryptophan residue in the h isoform. The pH and temperature dependence of the aggregation of both thioredoxins has been investigated. Our results indicate that the aggregation of TRX is highly dependent on pH and that the differences between the two TRX isoforms are linked to distinct pH dependencies. We have also analyzed the pH and temperature dependence of 12 distinct variants of TRX engineered by site-directed mutagenesis. The results obtained indicate that the differences in the hydrophobic core of the two TRX isoforms do not account for the differences of aggregation. On the other hand, we show the importance of His-109 as well as the second active site cysteine, Cys-39 in the aggregation mechanism.