biomolecules Review Structural and Functional Insights into Human Nuclear Cyclophilins Caroline Rajiv 1,2 and Tara L. Davis 1,3,* 1 Department of Biochemistry and Molecular Biology, Drexel University College of Medicine, Philadelphia, PA 19102, USA;
[email protected] 2 Janssen Pharmaceuticals Inc., 22-21062, 1400 McKean Rd, Spring House, PA 19477, USA 3 FORMA Therapeutics, 550 Arsenal St. Ste. 100, Boston, MA 02472, USA * Correspondence:
[email protected]; Tel.: +1-857-209-2342 Received: 31 October 2018; Accepted: 22 November 2018; Published: 4 December 2018 Abstract: The peptidyl prolyl isomerases (PPI) of the cyclophilin type are distributed throughout human cells, including eight found solely in the nucleus. Nuclear cyclophilins are involved in complexes that regulate chromatin modification, transcription, and pre-mRNA splicing. This review collects what is known about the eight human nuclear cyclophilins: peptidyl prolyl isomerase H (PPIH), peptidyl prolyl isomerase E (PPIE), peptidyl prolyl isomerase-like 1 (PPIL1), peptidyl prolyl isomerase-like 2 (PPIL2), peptidyl prolyl isomerase-like 3 (PPIL3), peptidyl prolyl isomerase G (PPIG), spliceosome-associated protein CWC27 homolog (CWC27), and peptidyl prolyl isomerase domain and WD repeat-containing protein 1 (PPWD1). Each “spliceophilin” is evaluated in relation to the spliceosomal complex in which it has been studied, and current work studying the biological roles of these cyclophilins in the nucleus are discussed. The eight human splicing complexes available in the Protein Data Bank (PDB) are analyzed from the viewpoint of the human spliceophilins. Future directions in structural and cellular biology, and the importance of developing spliceophilin-specific inhibitors, are considered. Keywords: peptidyl prolyl isomerases; nuclear cyclophilins; spliceophilins; alternative splicing; spliceosomes; NMR; X-ray crystallography 1.