RNF123

RNF123 Full Name

ring finger protein 123

RNF123 Introduction

RNF123 (ring finger protein 123), better known in the cell-cycle literature as KPC1 (Kip1 ubiquitylation-promoting complex 1), is an E3 ubiquitin ligase with two seemingly distinct but biologically connected jobs: controlling the degradation of the cyclin-dependent kinase inhibitor p27 and shaping the activity of the NF-kB transcription factor family. Together with its adaptor KPC2, RNF123 ubiquitinates p27 in the cytoplasm, targeting it for proteasomal destruction and thereby allowing cells to progress through the G1/S transition of the cell cycle. Because p27 is a major barrier to unscheduled proliferation, the RNF123-p27 axis is frequently co-opted in cancer, where elevated RNF123 activity promotes p27 loss and tumor growth. In parallel, RNF123 participates in the ubiquitin-dependent processing of the NF-kB1 precursor protein p105 into the p50 subunit, an event that alters the composition of NF-kB dimers and can shift cells between pro-inflammatory and tumor-suppressive gene expression programs. The dual control of a cell-cycle brake and an inflammatory transcription factor places RNF123 at the interface of proliferation, immunity, and cancer, making it an intriguing but still incompletely understood target for therapeutic intervention.

Figure 1. The structure of RNF123.

KPC1 Complex, Substrate Recognition, and Dual Mechanisms
The RNF123 protein carries an N-terminal RING finger that provides E3 ligase activity and additional domains that mediate association with its obligatory partner KPC2.
Together, RNF123 and KPC2 form the KPC complex, which is responsible for the cytoplasmic, ubiquitin- and proteasome-dependent degradation of p27Kip1 that occurs when quiescent cells are stimulated to re-enter the cell cycle.
Phosphorylation of p27 on specific residues in G1 promotes its export from the nucleus to the cytoplasm, where the KPC complex recognizes and ubiquitinates it, ensuring that p27 levels fall precisely when cyclin-dependent kinase activity is needed.
In a separate pathway, RNF123 promotes the ubiquitination of the p105 NF-kB1 precursor, which is then partially degraded by the proteasome to generate the mature p50 subunit.
Because p50 can partner with other NF-kB proteins or form repressive homodimers, RNF123-mediated p105 processing influences which NF-kB target genes are activated in a given context.
The RNF123 gene is located on chromosome 3, and the protein is expressed broadly, with activity regulated by the availability of KPC2 and by signaling pathways that control p27 localization.
The ability of a single E3 ligase to feed both a cell-cycle regulatory circuit and an inflammatory signaling pathway illustrates how ubiquitination integrates growth control with immune function.

Cell Cycle Control, p27 Loss in Cancer, and NF-kB Crosstalk
In proliferating cells, RNF123-mediated p27 degradation is essential for timely entry into S phase, and overexpression of a catalytically inactive RNF123 arrests cells with elevated p27 levels.
In human cancers, loss of p27 is common and often reflects accelerated proteolysis rather than gene deletion; increased expression or activity of the RNF123/KPC pathway has been implicated in this process in tumors such as breast, colon, and thyroid carcinomas.
Conversely, p27 also has cytoplasmic functions that influence cell migration, and RNF123-dependent relocation of p27 degradation to the cytoplasm may therefore affect both proliferation and invasion.
On the NF-kB side, RNF123-driven p105 processing can either amplify inflammatory responses, when p50 pairs with RelA, or restrain them, when repressive p50 homodimers predominate, depending on the cellular setting.
This bifunctional character means that RNF123 inhibition might simultaneously stabilize p27 (slowing proliferation) and alter inflammatory gene expression, an attractive combination for cancer therapy.
Small-molecule inhibitors of the RNF123-KPC2 interaction are being considered as a strategy to restore p27 and to rebalance NF-kB signaling in tumors, although specificity and on-target toxicity remain key challenges.

Alternate Names for RNF123

RNF123; ring finger protein 123; KPC1; FP1477; E3 ubiquitin-protein ligase RNF123; kip1 ubiquitination-promoting complex protein 1;