Ubiquitin protein ligase Nedd4 binds to connexin43 by a phosphorylation-modulated process

The TSG101 protein binds to connexins and is involved in connexin degradation

Gap junctions mediate electrical and metabolic communication between cells in almost all tissues and are proposed to play important roles in cellular growth control, differentiation and embryonic development. Gap junctional communication and channel assembly were suggested to be regulated by interaction of connexins with different proteins including kinases and phosphatases. Here, we identified the tumor susceptibility gene 101 (TSG101) protein to bind to the carboxyterminal tail of connexin45 in a yeast two-hybrid protein interaction screen. Glutathione S-transferase pull down experiments and immunoprecipitation revealed that not only connexin45 but also connexin30.2, -36, and -43 carboxyterminal regions were associated with TSG101 protein in pull down analyses and that connexin31, -43 and -45 co-precipitate with endogenous TSG101 protein in lysates from HM1 embryonic stem cells. TSG101 has been shown to be involved in cell cycle control, transcriptional regulation and turnover of endocytosed proteins. Thus, we decided to study the functional role of this interaction. SiRNA mediated knock down of TSG101 in HM1 embryonic stem cells led to increased levels of connexin43 and -45, prolonged half life of these connexins and increased transfer of microinjected Lucifer yellow. Our results suggest that TSG101 is involved in the degradation of connexins via interaction with connexin proteins.

Nedd4 augments the adaptive immune response by promoting ubiquitin-mediated degradation of Cbl-b in activated T cells

Nedd4 and Itch are E3 ubiquitin ligases that ubiquitinate similar targets in vitro and thus are thought to function similarly. T cells lacking Itch show spontaneous activation and T helper type 2 polarization. To test whether loss of Nedd4 affects T cells in the same way, we generated Nedd4(+/+) and Nedd4(-/-) fetal liver chimeras. Nedd4(-/-) T cells developed normally but proliferated less, produced less interleukin 2 and provided inadequate help to B cells. Nedd4(-/-) T cells contained more of the E3 ubiquitin ligase Cbl-b, and Nedd4 was required for polyubiquitination of Cbl-b induced by CD28 costimulation. Our data demonstrate that Nedd4 promotes the conversion of naive T cells into activated T cells. We propose that Nedd4 and Itch ubiquitinate distinct target proteins in vivo.

Lens fiber connexin turnover and caspase-3-mediated cleavage are regulated alternately by phosphorylation

Lens connexins are phosphorylated in vivo; however, the function and regulation of the phosphorylation remain largely unknown. We have previously identified an in vivo phosphorylation site, Ser(364), at the COOH terminus of lens connexin (Cx) Cx45.6 and phosphorylation appears to regulate connexin protein turnover. To assess the specific mechanism of Ser(364) phosphorylation in Cx45.6, exogenous wild type and Ser(364) mutant Cx45.6 were expressed in primary lens cultures through retroviral infection. Cx45.6 turnover was attenuated primarily by proteasomal inhibitors and to a lesser extent by lysosomal inhibitors. Furthermore, the level of Cx45.6 protein in ubiquitin co-expressed cells was significantly reduced as compared to the cells expressing Cx45.6 alone. Moreover, overexpression of ubiquitin led to a more significant decrease in wild type Cx45.6 than Cx45.6(S364A), a mutant deficient of phosphorylation site at Ser(364), although we did not detect any difference in the levels of ubiquitination between wild type and mutant Cx45.6. Interestingly, the mutant mimicking constitutive phosphorylation, Cx45.6(S364D), partially prevented the cleavage of Cx45.6 by caspase-3. Together, our data suggest that phosphorylation of Cx45.6 at Ser(364) appears to stimulate Cx45.6 turnover primarily through proteasome pathway and this phosphorylation inhibits the cleavage of Cx45.6 by caspase-3. These findings provide further insights into regulatory mechanism of the specific phosphorylation of connexins in the lens.

A novel connexin43-interacting protein, CIP75, which belongs to the UbL-UBA protein family, regulates the turnover of connexin43

The degradation of connexin43 (Cx43) has been reported to involve both lysosomal and proteasomal degradation pathways; however, very little is known about the mechanisms regulating these Cx43 degradation pathways. Using yeast two-hybrid, glutathione S-transferase pull-down, and co-immunoprecipitation approaches, we have identified a novel Cx43-interacting protein of approximately 75 kDa, CIP75. Laser confocal microscopy showed that CIP75 is located primarily at the endoplasmic reticulum, as indicated by the calnexin marker, with Cx43 co-localization in this perinuclear region. CIP75 belongs to the UbL (ubiquitin-like)-UBA (ubiquitin-associated) domain-containing protein family with a N-terminal UbL domain and a C-terminal UBA domain. The UBA domain of CIP75 is the main element mediating the interaction with Cx43, whereas the CIP75-interacting region in Cx43 resides in the PY motif and multiphosphorylation sites located between Lys 264 and Asn 302. Interestingly, the UbL domain interacts with the S2/RPN1 and S5a/RPN10 protein subunits of the regulatory 19 S proteasome cap subunit of the 26 S proteasome complex. Overexpression experiments suggested that CIP75 is involved in the turnover of Cx43 as measured by a significant stimulation of Cx43 degradation and reduction in its half-life with the opposite effects on Cx43 degradation observed in small interference RNA knockdown experiments.

Expression of the HPV11 E2 gene in transgenic mice does not result in alterations of the phenotypic pattern

The E2 early protein of human papillomaviruses (HPV) has been found associated with the mitotic spindle therefore being implicated in the partition of the replicated viral DNA to daughter cells. In addition, E2 proteins bind to the upstream regulatory region of the virus and to cellular promoters modulating thereby cellular transcription and differentiation. In many cervical cancers, the E2 reading frame is interrupted upon incorporation of the viral genome into the host DNA. This results in the loss of the E2 mediated transcriptional repression and uncontrolled expression of the viral oncogenes. All these results have been obtained in transfected cells but no information is available on the E2 effects in the context of the entire organism. Transgenic mice were generated expressing the E2 protein of HPV11 under the control of the Ubiquitin C promoter. E2 mRNA is present in all mice tissues analysed and the E2 protein expressed in the skin (the target tissue of HPV11) was shown by Western blotting, albeit at a very low level. Analysis of the transgenic mice shows no major histological changes in the skin or all other tissues investigated. These data indicate that in transgenic mice the human papillomavirus type 11 E2 does not grossly modulate cellular proliferation or differentiation events.

C-Terminal Truncation of Connexin43 Changes Number, Size, and Localization of Cardiac Gap Junction Plaques

Haplodeficient mice expressing carboxyl-terminally truncated Cx43 (K258stop/KO), instead of the wild-type Cx43 isoform, reach adulthood and reveal no abnormalities in heart morphology. Here, we have analyzed the expression of K258stop protein and the morphology of gap junctions in adult hearts of these mice. Coimmunofluorescence analysis revealed reduced juxtaposition of K258stop with other junctional proteins at the intercalated disc. Immunoprecipitation studies documented changes in the interaction with previously described Cx43 binding proteins. Quantitative transmission electron and confocal microscopy confirmed the localization of K258stop gap junctions to the periphery of the intercalated disc and further revealed an increase in the size of K258stop gap junction plaques and a reduction in their number. Dual whole cell patch clamp analysis confirmed that K258stop gap junctions were functional, with single channel properties similar to those described in exogenous systems. We conclude that the carboxyl-terminal domain of Cx43 (Cx43CT) is involved in regulating the localization, number and size of Cx43 plaques in vivo. Conversely, protein interactions or posttranslational modifications taking place within the Cx43CT are not required for the assembly of functional gap junctions in the intercalated disc.

Specificity and autoregulation of Notch binding by tandem WW domains in suppressor of Deltex

WW domains target proline-tyrosine (PY) motifs and frequently function as tandem pairs. When studied in isolation, single WW domains are notably promiscuous and regulatory mechanisms are undoubtedly required to ensure selective interactions. Here, we show that the fourth WW domain (WW4) of Suppressor of Deltex, a modular Nedd4-like protein that down-regulates the Notch receptor, is the primary mediator of a direct interaction with a Notch-PY motif. A natural Trp to Phe substitution in WW4 reduces its affinity for general PY sequences and enhances selective interaction with the Notch-PY motif via compensatory specificity-determining interactions with PY-flanking residues. When WW4 is paired with WW3, domain-domain association, impeding proper folding, competes with Notch-PY binding to WW4. This novel mode of autoinhibition is relieved by binding of another ligand to WW3. Such cooperativity may facilitate the transient regulatory interactions observed in vivo between Su(dx) and Notch in the endocytic pathway. The highly conserved tandem arrangement of WW domains in Nedd4 proteins, and similar arrangements in more diverse proteins, suggests domain-domain communication may be integral to regulation of their associated cellular activities.

Ubiquitination of gap junction proteins

Gap junctions are plasma membrane domains containing arrays of channels that exchange ions and small molecules between neighboring cells. Gap junctional intercellular communication enables cells to directly cooperate both electrically and metabolically. Several lines of evidence indicate that gap junctions are important in regulating cell growth and differentiation and for maintaining tissue homeostasis. Gap junction channels consist of a family of transmembrane proteins called connexins. Gap junctions are dynamic structures, and connexins have a high turnover rate in most tissues. Connexin43 (Cx43), the best-studied connexin isoform, has a half-life of 1.5-5 h; and its degradation involves both the lysosomal and proteasomal systems. Increasing evidence suggests that ubiquitin is important in the regulation of Cx43 endocytosis. Ubiquitination of Cx43 is thought to occur at the plasma membrane and has been shown to be regulated by protein kinase C and the mitogen-activated protein kinase pathway. Cx43 binds to the E3 ubiquitin ligase Nedd4, in a process modulated by Cx43 phosphorylation. The interaction between Nedd4 and Cx43 is mediated by the WW domains of Nedd4 and involves a proline-rich sequence conforming to a PY (XPPXY) consensus motif in the C terminus of Cx43. In addition to the PY motif, an overlapping tyrosine-based sorting signal conforming to the consensus of an YXXphi motif is involved in Cx43 endocytosis, indicating that endocytosis of gap junctions involves both ubiquitin-dependent and -independent pathways. Here, we discuss current knowledge on the ubiquitination of connexins.

Gap junctional complexes: From partners to functions

Gap junctions (GJ), specialised membrane structures that mediate cell-to-cell communication in almost all animal tissues, are composed of intercellular channel-forming integral membrane proteins termed connexins (Cxs), innexins or pannexins. The activity of these channels is closely regulated, particularly by intramolecular modifications as phosphorylation of proteins, via the formation of multiprotein complexes where pore-forming subunits bind to auxiliary channel subunits and associate with scaffolding proteins that play essential roles in channel localization and activity. Scaffolding proteins link signalling enzymes, substrates, and potential effectors (such as channels) into multiprotein signalling complexes that may be anchored to the cytoskeleton. Protein-protein interactions play essential roles in channel localization and activity and, besides their cell-to-cell channel-forming functions, gap junctional proteins now appear involved in different cellular functions (e.g. transcriptional and cytoskeletal regulation). The present review summarizes the recent progress regarding the proteins capable of interacting with junctional proteins and their functional importance.