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.
Highlights
A peptide bond in a folded protein can attain either cis or trans conformation, with the latter being favored due to geometrical and thermodynamic parameters (Ramachandran and Sasisekharan, 1968)
The chaperonic function of an Arabidopsis cyclophilin AtCYP40 (CYP40) was shown to be independent of peptidyl-prolyl cis-trans isomerase (PPIase) activity since the enzymatically inactive mutants of AtCYP40 were able to facilitate the assembly of RNA induced silencing complex (RISC; Iki et al, 2012)
The Arabidopsis cyclophilin AtCYP59, which besides PPIase domain contains an RNA Recognition Motif (RRM) motif, a Zn-knuckle and a charged C-terminal domain consisting of RS/RD repeats, was proposed to regulate transcription through its interaction with the immature mRNA (Gullerova et al, 2006; Bannikova et al, 2012)
Summary
A peptide bond in a folded protein can attain either cis or trans conformation, with the latter being favored due to geometrical and thermodynamic parameters (Ramachandran and Sasisekharan, 1968). Site-directed mutagenesis studies provided evidence that PPIase activity of TaCYPA-1 is regulated through a dual mechanism involving loop displacement (Kaur et al, 2017), as observed in the divergent cyclophilin CsCYP (Campos et al, 2013), and by the interaction between Cys122 and Cys126, as reported for the non-divergent SmCYPA
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