PCNP is a PEST-containing nuclear protein that is ubiquitinated by NIRF, a Np95/ICBP90-like RING finger protein probably involved in cell cycle regulation []. PEST sequences, which are rich in proline (P), glutamic acid (E), serine (S) and threonine (T), are found in a number of short-lived proteins, such as transcription factors and cell cycle-associated proteins. Their function is generally controlled by proteolysis, mostly via ubiquitin-mediated degradation []. NIRF and PCNP may constitute a novel signaling pathway with some relation to cell proliferation.
