Description
Background: PPIG belongs to a highly conserved class of cyclophilins that function as peptidyl-prolyl-isomerases (PPIases) to catalyze the conversion of cis-proline to trans-proline in a polypeptide chain (1-4). PPIG contains an amino-terminal cyclophilin domain followed by Nopp140 repeats that are involved in its function as a nuclear chaperone (5). The carboxy-terminal of PPIG contains a SR (arginine-serine dipeptide repeat) domain (3,4) that is involved in pre-mRNA splicing and processing (6). PPIG interacts with the carboxy-terminal domain of RNA polymerase II as well as several other SR family splicing factors. These interactions lead to changes in localization and conformation and suggest a regulatory role in transcription and pre-mRNA splicing in the elongating RNA polymerase complex (7,8). PPIG is found in the nuclear matrix and nuclear speckles and is involved in the regulation of gene expression. PPIG shows a predominantly diffuse cytoplasmic distribution at the onset of mitosis, and in late telophase the isomerase is recruited to the newly formed nuclei (9)