## 4 Discussion  
Because plants are sessile organisms, protection from temperature fluctuations is critical for their survival. Thus, plants have developed a number of tools or systems to combat environmental stresses. Among these, molecular chaperone proteins that support the structural assembly or disassembly of intracellular target substrates and maintain their conformational folding contribute to the protection of plants from diverse external stresses. Most of the HSPs, including HSP60, HSP70, HSP90, and small HSPs, are distributed in all living organisms, ranging from bacteria to humans, and function as chaperones. The expression of genes encoding these HSPs is induced by diverse environmental factors, including cold, chemical treatment, wounding, and ultraviolet (UV) light. HSPs not only stabilize newly synthesized proteins to ensure correct folding but also repair misfolded proteins damaged by internal or external stresses. As the name implies, HSPs primarily function under heat shock by stabilizing heat stress-denatured molten-globular target substrates and renaturing these proteins to form native structures after the stresses are removed [28; 29]. To mediate interaction between the partially denatured and unstructured proteins/peptides and to prevent the self-aggregation of substrates, chaperone proteins contain hydrophobic surfaces. The TPR and ANK domains are highly optimized to perform this function since they have an irregular structure with a large proportion of hydrophobic residues. Based on these structural characteristics, the TPR domain is reported to function as a co-chaperone in association with HSP90/70 [37; 38; 39]. Other _Arabidopsis_ proteins such as protein phosphatase 5 (PP5) [34] and the Trx domain-containing protein AtTDX, which functions as a disulfide reductase [21], also contain TPR domains. In these proteins, TPR domains are involved in protein-protein interactions, protein assembly, and signal transduction. Overexpression of genes encoding TPR domain-containing proteins has been shown to enhance heat shock tolerance in plants [40; 41]. Similarly, the ANK domain first discovered in the yeast Cdc10 protein contains 4-6 repeats of a 33 aa motif, forming two \(\alpha\)-helices and a separate type 1 \(\beta\)-bulge loop [14; 42]. The lateral surface structure of these repeats is hydrophobic with changeable conformation, which is mainly responsible for protein-protein interactions. The ANK domain forms a linear solenoid structure and performs multiple functions, including cell cycle control, ion transport, and signal transduction via protein-protein interactions. The ANK domain is present in several important proteins including the non-expressor of PR1 (NPR1), which functions as a key regulator of systemic acquired resistance (SAR) [43; 44]; accelerated cell death 6 protein (ACD6), which contains one ANK motif and regulates plant defense via salicylic acid signaling [45]; and the nuclear-located ANK protein in tobacco, which enhances the auxin-mediated bZIP transcription factors, BZI-1 and BZI-2, through its regulation of protein complex formation [46]. These proteins also participate in pathogen resistance and hormone signaling [44; 45; 46]. In addition, the ANK repeat-containing protein 2A (AKR2A) protects its targets from stress-induced aggregation. In _Arabidopsis_, APX3 and the outer envelope protein 7 (OEP7) work together in chloroplasts to maintain plant growth and development [47]. In humans, mutation of \(ANK\) genes has a critical effect on the association of the encoded protein with the cell cycle inhibitor p16 or Notch protein, resulting in diverse diseases such as cancer, neurological disorders including alcoholism, nicotine dependency, obesity, and juvenile delinquency [14; 48; 49]. Thus, investigation of ANKs and TPRs is highly important to understand the regulation of their protein-protein interaction activity, identify the crucial residues needed for signal transduction, and control their chaperone function in plants.