Plant Cyclophilins: Multifaceted Proteins With Versatile Roles

Abstract

Cyclophilins constitute a family of ubiquitous proteins that bind cyclosporin A (CsA), an immunosuppressant drug. Several of these proteins possess peptidyl-prolyl cis-trans isomerase (PPIase) activity that catalyzes the cis-trans isomerization of the peptide bond preceding a proline residue, essential for correct folding of the proteins. Compared to prokaryotes and other eukaryotes studied until now, the cyclophilin gene families in plants exhibit considerable expansion. With few exceptions, the role of the majority of these proteins in plants is still a matter of conjecture. However, recent studies suggest that cyclophilins are highly versatile proteins with multiple functionalities, and regulate a plethora of growth and development processes in plants, ranging from hormone signaling to the stress response. The present review discusses the implications of cyclophilins in different facets of cellular processes, particularly in the context of plants, and provides a glimpse into the molecular mechanisms by which these proteins fine-tune the diverse physiological pathways.

Keywords: FKBP; cyclophilin; hormones; immunophilins; peptidyl-prolyl cis-trans isomerase; stress.

FIGURE 1

Comparative analysis of domain architecture of Arabidopsis cyclophilins with their orthologs in human and Saccharomyces cerevisiae. The amino acid residues that define the protein domains are designated according to Galat (2004), Wang and Heitman (2005), Kumari et al. (2015), and Schiene-Fischer (2015). For Arabidopsis cyclophilins that may have alternatively spliced forms, the domain architecture is shown for only a single variant. CLD, cyclophilin-like domain; RRM, RNA recognition motif; TPR, tetratricopeptide repeat; U-box, U box domain; WD40, WD40 repeat; RanBDl, Ran binding protein 1 domain; zf RanBP, Zn-finger, Ran-binding; SR, Serine arginine rich domain. The nomenclature and alternative protein names are given in the box. Scale bar represents the length of amino acid sequence.

FIGURE 2

Role of cyclophilins in regulation of Auxin-responsive genes. At low levels of Auxin, Aux/IAA proteins bind to auxin response factors (ARFs) directly or through recruitment of transcriptional corepressor such as TOPLESS (TPL) and inhibit their activity (1). When present at high levels, the auxin binds to its receptor TRANSPORT INHIBITOR RESPONSE1 (TIR1) and enhances its interaction with the highly conserved ‘degron’ motif GWPPV in domain II of Aux/IAAs (2 and 3). The Aux/IAA proteins bind to SCFTIR1-Auxin complex only when W104-P105 isomer in the ‘degron’ motif GWPPV (residues 103, 104, 105, 106 and 107, respectively, in rice) is in cis conformation. The trans conformer of W-P (T105-T106) in the ‘degron’ motif is catalyzed to cis form (c105-T106) by OsCYP2 (4). The Aux/IAA-SCFTIR1 complex leads to ubiquitination of Aux/IAA proteins (5), which are then degraded by 26S Proteasome (6), leading to expression of Auxin responsive genes including Aux/IAA (7) (Adapted from Tan et al., 2007; Mockaitis and Estelle, 2008; Jing et al., 2015; Created with BioRender.com).