Crystal structure of the human U4/U6 small nuclear ribonucleoprotein particle-specific SnuCyp-20, a nuclear cyclophilin

The cyclophilin SnuCyp-20 is a specific component of the human U4/U6 small nuclear ribonucleoprotein particle involved in the nuclear splicing of pre-mRNA. It stably associates with the U4/U6-60kD and -90kD proteins, the human orthologues of the Saccharomyces cerevisiae Prp4 and Prp3 splicing factors. We have determined the crystal structure of SnuCyp-20 art 2.0-Angstrom resolution by molecular replacement. Tbe structure of SnuCyp-20 closely resembles that of human cyclophilin A (hCypA), In particular, the catalytic centers of SnuCyp-20 and hCypA superimpose perfectly, which is reflected by the observed peptidyl-prolyl-cis/trans-isomerase activity of SnuCyp-20. The surface properties of both proteins, however, differ significantly. Apart from seven additional amino-terminal residues, the insertion of five amino acids in the loop alpha 1-beta 3 and of one amino acid in the loop alpha 2-beta 8 changes the conformations of both loops. The enlarged loop alpha 1-beta 3 is involved in the formation of a wide cleft with predominantly hydrophobic character, We propose that this enlarged loop is required for the interaction with the U4/U6-60kD protein.