Cullin 7 mediates proteasomal and lysosomal degradations of rat Eag1 potassium channels

Abstract

Mammalian Eag1 (Kv10.1) potassium (K+) channels are widely expressed in the brain. Several mutations in the gene encoding human Eag1 K+ channel have been associated with congenital neurodevelopmental anomalies. Currently very little is known about the molecules mediating protein synthesis and degradation of Eag1 channels. Herein we aim to ascertain the protein degradation mechanism of rat Eag1 (rEag1). We identified cullin 7 (Cul7), a member of the cullin-based E3 ubiquitin ligase family, as a novel rEag1 binding partner. Immunoprecipitation analyses confirmed the interaction between Cul7 and rEag1 in heterologous cells and neuronal tissues. Cul7 and rEag1 also exhibited significant co-localization at synaptic regions in neurons. Over-expression of Cul7 led to reduced protein level, enhanced ubiquitination, accelerated protein turn-over, and decreased current density of rEag1 channels. We provided further biochemical and morphological evidence suggesting that Cul7 targeted endoplasmic reticulum (ER)- and plasma membrane-localized rEag1 to the proteasome and the lysosome, respectively, for protein degradation. Cul7 also contributed to protein degradation of a disease-associated rEag1 mutant. Together, these results indicate that Cul7 mediates both proteasomal and lysosomal degradations of rEag1. Our findings provide a novel insight to the mechanisms underlying ER and peripheral protein quality controls of Eag1 channels.

The functional analysis of Cullin 7 E3 ubiquitin ligases in cancer

“…Recently, Sun et al revealed that CD36 could negatively regulate insulin activation, indicating that CD36 interacted with IRS-1, thereby abrogating the binding between IRS-1 and CUL7, which would further increase IRS-1 stability and affect insulin signaling 65 and might ultimately result in tumor suppression. In addition, the level of the voltage-gated potassium channel Eag2, which promotes cell migration in medulloblastoma 66 , 67 , is dramatically decreased by CUL7 overexpression 68 , which might also suppress the progression of tumors. Therefore, CUL7 may play a suppressive role in tumor growth and metastasis.…”

“…Human Eag K + channel mutations exist in diseases characterized by congenital neurodevelopmental anomalies 111 – 114 . Hsu et al found that CUL7 was a novel binding partner of rat Eag1 and targeted the rEag1 potassium channel for proteasomal- and lysosomal degradation 68 . However, whether F-box proteins mediate the degradation of rEag1 has not been reported.…”

FKBP8 Enhances Protein Stability of the CLC-1 Chloride Channel at the Plasma Membrane

“…Instead, we employed brefeldin A (BFA), which inhibits forward trafficking of proteins from the ER to the Golgi [47], to investigate the effect of FKBP8 co-expression on protein stability of plasma membrane-resident CLC-1. In other words, as a result of BFA-induced blockade of forward trafficking, the time course of the reduction in surface biotinylated CLC-1 protein in response to increasing BFA treatment durations reflects the turnover rate of CLC-1 channels at the plasma membrane, a quantitative analysis known as BFA chase assay [48]. Figure 6 exemplifies the BFA chase response of surface CLC-1 protein: in the presence of FKBP8, the estimated protein half-life of surface CLC-1 dramatically increased from about 8.4 to about 14.8 h, implying that FKBP8 may effectively enhance protein stability of CLC-1 at the plasma membrane.…”

CUL7 promotes cancer cell survival through promoting Caspase‐8 ubiquitination

“…CUL7, OBSL1, and CCDC8 form a centrosome complex that inhibits CUL9‐mediated Survivin ubiquitination and degradation, thereby linking mitosis to cell survival . Furthermore, CUL7 targets the rat Eag1 potassium channel to proteasomal‐ and lysosomal degradation …”

“…13 Furthermore, CUL7 targets the rat Eag1 potassium channel to proteasomal-and lysosomal degradation. 14 CUL7 functions as an oncogene. High CUL7 expression levels have poor clinical outcomes and survival rates in several types of cancers.…”