Structural determinants underlie the essentiality of the m-type thioredoxin TrxA in Synechocystis

Thioredoxins, crucial regulators of thiol-based redox signaling in photosynthetic organisms, come in various isoforms. Among these, TrxA, the cyanobacterial counterpart of chloroplast m-type thioredoxins, stands out as the sole thioredoxin vital for cyanobacteria's survival. The reason for TrxA's essentiality, however, remains uncertain, leading researchers to investigate whether it is due to its high abundance within cells or its unique molecular characteristics.

To address this, scientists evaluated whether other thioredoxins could replace TrxA's role and used Chimeras of TrxA and TrxB to pinpoint structural elements that contribute to its specific functionality. Their findings showed that enhancing TrxB, TrxQ, or TrxC did not result in a successful replacement of TrxA, even though these proteins accumulated to similar levels as endogenous TrxA. Similarly, employing NTRC from another organism did not rescue the effects of reduced TrxA levels.

A detailed structural analysis of TrxA in comparison to other thioredoxins revealed differences in surface electrostatics and structural features surrounding its catalytic site. Further examination of TrxB/TrxA Chimeras indicated that the determinants crucial for functional replacement are primarily located within TrxA's C-terminal region.

Additionally, these chimeras differed in their ability to activate enzymes involved in sugar metabolism, both in cyanobacteria and pea plants, suggesting that this region plays a key role in specific interactions with target molecules.

In conclusion, the study reveals that TrxA's essential function cannot be attributed solely to its high abundance in cells. Instead, the protein's specific molecular features, particularly those found in its C-terminal region, are necessary for its essential role in maintaining cellular processes.