Crystal structures of two functionally different thioredoxins in spinach chloroplasts

Area of Science:

• Biochemistry
• Structural Biology
• Plant Physiology

Background:

• Thioredoxins are essential redox proteins regulating cellular processes.
• Chloroplasts utilize two distinct thioredoxins, f (Trx f) and m (Trx m), with different origins and functions.
• Trx f activates Calvin cycle enzymes, while Trx m targets other chloroplast enzymes.

Purpose of the Study:

• To elucidate the structure-function relationships of chloroplast thioredoxins f and m.
• To understand the molecular basis for their distinct substrate specificities.

Main Methods:

• X-ray crystallography was employed to determine the structures of Trx f and Trx m.
• Structures were solved for oxidized and reduced forms, and for different Trx f variants.
• High-resolution structures (2.1-2.3 Å) were obtained.

Main Results:

• Thioredoxin f typically forms a monomer, while truncated Trx f and Trx m form dimers.
• Both proteins share the conserved thioredoxin fold but differ in surface topology and active site charge distribution.
• Trx f possesses an additional N-terminal alpha-helix and a unique exposed cysteine residue.

Conclusions:

• The structural differences, particularly in surface topology and active site flexibility, underlie the specificities of Trx f and Trx m.
• These findings provide insights into the regulatory mechanisms of photosynthesis.