The ArfGEF GBF-1 Is Required for ER Structure,Secretion and Endocytic Transport in C. elegans

Small GTPases of the Sar/Arf family are essential to generate transport containers that mediate communication between organelles of the secretory pathway. Guanine nucleotide exchange factor (GEFs) activate the small GTPases and help their anchorage in the membrane. Thus, GEFs in a way temporally and spatially control Sar1/Arf1 GTPase activation. We investigated the role of the ArfGEF GBF-1 in C. elegans oocytes and intestinal epithelial cells. GBF-1 localizes to the cis-Golgi and is part of the t-ER-Golgi elements. GBF-1 is required for secretion and Golgi integrity. In addition, gbf-1(RNAi) causes the ER reticular structure to become dispersed, without destroying ER exit sites (ERES) because the ERES protein SEC-16 was still localized in distinct punctae at t-ER-Golgi units. Moreover, GBF-1 plays a role in receptor-mediated endocytosis in oocytes, without affecting recycling pathways. We find that both the yolk receptor RME-2 and the recycling endosome-associated RAB-11 localize similarly in control and gbf-1(RNAi) oocytes. While RAB5-positive early endosomes appear to be less prominent and the RAB-5 levels are reduced by gbf-1(RNAi) in the intestine, RAB-7-positive late endosomes were more abundant and formed aggregates and tubular structures. Our data suggest a role for GBF-1 in ER structure and endosomal traffic.     

Cytoplasmic Amino Acids within the Membrane Interface Region Influence Connexin Oligomerization

Gap junction channels composed of connexins connect cells, allowing intercellular communication. Their cellular assembly involves a unique quality-control pathway. Some connexins [including connexin43 (Cx43) and Cx46] oligomerize in the trans-Golgi network following export of stabilized monomers from the endoplasmic reticulum (ER). In contrast, other connexins (e.g., Cx32) oligomerize early in the secretory pathway. Amino acids near the cytoplasmic aspect of the third transmembrane domain have previously been shown to determine this difference in assembly sites. Here, we characterized the oligomerization of two connexins expressed prominently in the vasculature, Cx37 and Cx40, using constructs containing a C-terminal dilysine-based ER retention/retrieval signal (HKKSL) or treatment with brefeldin A to block ER vesicle trafficking. Both methods led to intracellular retention of connexins, since the cells lacked gap junction plaques. Retention of Cx40 in the ER prevented it from oligomerizing, comparable to Cx43. By contrast, ER-retained Cx37 was partially oligomerized. Replacement of two amino acids near the third transmembrane domain of Cx43 (L152 and R153) with the corresponding amino acids from Cx37 (M152 and G153) resulted in early oligomerization in the ER. Thus, residues that allow Cx37 to oligomerize early in the secretory pathway could restrict its interactions with coexpressed Cx40 or Cx43 by favoring homomeric oligomerization, providing a structural basis for cells to produce gap junction channels with different connexin composition.     

Plant Sar1 isoforms with near-identical protein sequences exhibit different localisations and effects on secretion

In plants, differentiation of subdomains of the endoplasmic reticulum (ER) dedicated to protein export, the ER export sites (ERES), is influenced by the type of export-competent membrane cargo to be delivered to the Golgi. This raises a fundamental biological question: is the formation of transport intermediates at the ER for trafficking to the Golgi always regulated in the same manner? To test this, we followed the distribution and activity of two plant Sar1 isoforms. Sar1 is the small GTPase that regulates assembly of COPII (coat protein complex II) on carriers that transport secretory cargo from ER to Golgi. We show that, in contrast to a tobacco Sar1 isoform, the two Arabidopsis Sar1 GTPases were localised at ERES, independently of co-expression of Golgi-destined membrane cargo in tobacco cells. Although both isoforms labelled ERES, one was found to partition with the membrane fraction to a greater extent. The different distribution of fluorescent fusions of the two isoforms was influenced by the nature of an amino acid residue at the C-terminus of the protein, suggesting that the requirements for membrane association of the two GTPases are not equal. Furthermore, functional analyses based on the secretion of the bulk flow marker α-amylase indicated that over-expression of GTP-restricted mutants of the two isoforms caused different levels of ER export inhibition. These novel results indicate a functional heterogeneity among plant Sar1 isoforms.     

ERp29 Restricts Connexin43 Oligomerization in the Endoplasmic Reticulum

Connexin43 (Cx43) is a gap junction protein that forms multimeric channels that enable intercellular communication through the direct transfer of signals and metabolites. Although most multimeric protein complexes form in the endoplasmic reticulum (ER), Cx43 seems to exit from the ER as monomers and subsequently oligomerizes in the Golgi complex. This suggests that one or more protein chaperones inhibit premature Cx43 oligomerization in the ER. Here, we provide evidence that an ER-localized, 29-kDa thioredoxin-family protein (ERp29) regulates Cx43 trafficking and function. Interfering with ERp29 function destabilized monomeric Cx43 oligomerization in the ER, caused increased Cx43 accumulation in the Golgi apparatus, reduced transport of Cx43 to the plasma membrane, and inhibited gap junctional communication. ERp29 also formed a specific complex with monomeric Cx43. Together, this supports a new role for ERp29 as a chaperone that helps stabilize monomeric Cx43 to enable oligomerization to occur in the Golgi apparatus.     

Gap junction turnover, intracellular trafficking, and phosphorylation of connexin43 in brefeldin A-treated rat mammary tumor cells

Intercellular gap junction channels are thought to form when oligomers of connexins from one cell (connexons) register and pair with connexons from a neighboring cell en route to forming tightly packed arrays (plaques). In the current study we used the rat mammary BICR-M1Rk tumor cell line to examine the trafficking, maturation, and kinetics of connexin43 (Cx43). Cx43 was conclusively shown to reside in the Golgi apparatus in addition to sites of cell-cell apposition in these cells and in normal rat kidney cells. Brefeldin A (BFA) blocked Cx43 trafficking to the surface of the mammary cells and also prevented phosphorylation of the 42-kD form of Cx43 to 44- and 46-kD species. However, phosphorylation of Cx43 occurred in the presence of BFA while it was still a resident of the ER or Golgi apparatus yielding a 43-kD form of Cx43. Moreover, the 42- and 43-kD forms of Cx43 trapped in the ER/Golgi compartment were available for gap junction assembly upon the removal of BFA. Mammary cells treated with BFA for 6 h lost preexisting gap junction "plaques," as well as the 44- and 46-kD forms of Cx43 and functional coupling. These events were reversible 1 h after the removal of BFA and not dependent on protein synthesis. In summary, we provide strong evidence that in BICR-M1Rk tumor cells: (a) Cx43 is transiently phosphorylated in the ER/Golgi apparatus, (b) Cx43 trapped in the ER/Golgi compartment is not subject to rapid degradation and is available for the assembly of new gap junction channels upon the removal of BFA, (c) the rapid turnover of gap junction plaques is correlated with the loss of the 44- and 46-kD forms of Cx43.     

The Noncanonical Functions of Cx43 in the Heart

There is abundant evidence showing that connexins form gap junctions. Yet this does not exclude the possibility that connexins can exert other functions, separate from that of gap junction (or even a permeable hemichannel) formation. Here, we focus on these noncanonical functions of connexin43 (Cx43), particularly in the heart. We describe two specific examples: the importance of Cx43 on intercellular adhesion, and the role of Cx43 in the function of the sodium channel. We propose that these two functions of Cx43 have important repercussions on the propagation of electrical activity in the heart, irrespective of the presence of permeable gap junction channels. Overall, the gap junction–independent functions of Cx43 in cardiac electrophysiology emerge as an exciting area of future research.     

Autophagosome Formation Depends on the Small GTPase Rab1 and Functional ER Exit Sites

Autophagy is an important cellular degradation pathway present in all eukaryotic cells. Via this pathway, portions of the cytoplasm and/or organelles are sequestered in double‐membrane structures called autophagosomes. In spite of the significant advance achieved in autophagy, the long‐standing question about the source of the autophagic membrane remains unsolved. We have investigated the role of the secretory pathway in autophagosome biogenesis. Sar1 and Rab1b are monomeric GTPases that control traffic from the endoplasmic reticulum (ER) to the Golgi. We present evidence indicating that the activity of both proteins is required for autophagosome formation. Overexpression of dominant‐negative mutants and the use of siRNAs impaired autophagosome generation as determined by LC3 puncta formation and light chain 3 (LC3)‐II processing. In addition, our results indicate that the autophagic and secretory pathways intersect at a level preceding the brefeldin A blockage, suggesting that the transport from the cis/medial Golgi is not necessary for autophagosome biogenesis. Our present results highlight the role of transport from the ER in the initial events of the autophagic vacuole development.     

Sequential coupling between COPII and COPI vesicle coats in endoplasmic reticulum to Golgi transport

COPI and COPII are vesicle coat complexes whose assembly is regulated by the ARF1 and Sar1 GTPases, respectively. We show that COPI and COPII coat complexes are recruited separately and independently to ER (COPII), pre-Golgi (COPI, COPII), and Golgi (COPI) membranes of mammalian cells. To address their individual roles in ER to Golgi transport, we used stage specific in vitro transport assays to synchronize movement of cargo to and from pre-Golgi intermediates, and GDP- and GTP-restricted forms of Sar1 and ARF1 proteins to control coat recruitment. We find that COPII is solely responsible for export from the ER, is lost rapidly following vesicle budding and mediates a vesicular step required for the build-up of pre-Golgi intermediates composed of clusters of vesicles and small tubular elements. COPI is recruited onto pre-Golgi intermediates where it initiates segregation of the anterograde transported protein vesicular stomatitis virus glycoprotein (VSV-G) from the retrograde transported protein p58, a protein which actively recycles between the ER and pre-Golgi intermediates. We propose that sequential coupling between COPII and COPI coats is essential to coordinate and direct bi-directional vesicular traffic between the ER and pre-Golgi intermediates involved in transport of protein to the Golgi complex.     

Genetic interactions in yeast between Ypt GTPases and Arf guanine nucleotide exchangers.

Two families of GTPases, Arfs and Ypt/rabs, are key regulators of vesicular transport. While Arf proteins are implicated in vesicle budding from the donor compartment, Ypt/rab proteins are involved in the targeting of vesicles to the acceptor compartment. Recently, we have shown a role for Ypt31/32p in exit from the yeast trans-Golgi, suggesting a possible function for Ypt/rab proteins in vesicle budding as well. Here we report the identification of a new member of the Sec7-domain family, SYT1, as a high-copy suppressor of a ypt31/32 mutation. Several proteins that belong to the Sec7-domain family, including the yeast Gea1p, have recently been shown to stimulate nucleotide exchange by Arf GTPases. Nucleotide exchange by Arf GTPases, the switch from the GDP- to the GTP-bound form, is thought to be crucial for their function. Sec7p itself has an important role in the yeast secretory pathway. However, its mechanism of action is not yet understood. We show that all members of the Sec7-domain family exhibit distinct genetic interactions with the YPT genes. Biochemical assays demonstrate that, although the homology between the members of the Sec7-domain family is relatively low (20-35%) and limited to a small domain, they all can act as guanine nucleotide exchange factors (GEFs) for Arf proteins, but not for Ypt GTPases. The Sec7-domain of Sec7p is sufficient for this activity. Interestingly, the Sec7 domain activity is inhibited by brefeldin A (BFA), a fungal metabolite that inhibits some of the Arf-GEFs, indicating that this domain is a target for BFA. These results demonstrate that the ability to act as Arf-GEFs is a general property of all Sec7-domain proteins in yeast. The genetic interactions observed between Arf GEFs and Ypt GTPases suggest the existence of a Ypt-Arf GTPase cascade in the secretory pathway.     

Lysosomal and proteasomal degradation play distinct roles in the life cycle of Cx43 in gap junctional intercellular communication-deficient and -competent breast tumor cells

The present study was designed to determine the specific roles played by lysosomes and proteasomes in the degradation of Cx43 in both gap junctional intercellular communication-deficient MDA-MB-231 and -competent BICR-M1Rk cells. In MDA-MB-231 cells, immunolocalization and brefeldin A protein transport blocking studies revealed that there was a propensity for newly synthesized Cx43 to be transported to lysosomes. On the other hand, light and electron microscopic analysis of BICR-M1Rk cells showed that Cx43 gap junctions were prevalent with a subpopulation of intracellular Cx43 localized to lysosomes. In both cell types, Western blots revealed a notable increase in total cellular Cx43 in response to lysosome inhibitors. Interestingly, lactacystin inhibition of proteosomal degradation in MDA-MB-231 cells resulted in a marked increase in phosphorylated Cx43 at the expense of non-phosphorylated Cx43, and this change corresponded with an increase in "oversized" gap junction plaques. In BICR-M1Rk cells, lactacystin treatment partially prevented the BFA-induced loss of gap junctions. Together, our data suggests that lysosomes play a key role in not only degrading internalized gap junction in BICR-M1Rk cells but also in degrading Cx43 delivered from early secretory compartments to lysosomes in MDA-MB-231 cells. Overall proteasomal degradation regulates the stability of phosphorylated Cx43 and appears to promote the internalization of Cx43 from the cell surface.     

Evidence that the entire Golgi apparatus cycles in interphase HeLa cells: sensitivity of Golgi matrix proteins to an ER exit block.

We tested whether the entire Golgi apparatus is a dynamic structure in interphase mammalian cells by assessing the response of 12 different Golgi region proteins to an endoplasmic reticulum (ER) exit block. The proteins chosen spanned the Golgi apparatus and included both Golgi glycosyltransferases and putative matrix proteins. Protein exit from ER was blocked either by microinjection of a GTP-restricted Sar1p mutant protein in the presence of a protein synthesis inhibitor, or by plasmid-encoded expression of the same dominant negative Sar1p. All Golgi region proteins examined lost juxtanuclear Golgi apparatus-like distribution as scored by conventional and confocal fluorescence microscopy in response to an ER exit block, albeit with a differential dependence on Sar1p concentration. Redistribution of GalNAcT2 was more sensitive to low Sar1p(dn) concentrations than giantin or GM130. Redistribution was most rapid for p27, COPI, and p115. Giantin, GM130, and GalNAcT2 relocated with approximately equal kinetics. Distinct ER accumulation could be demonstrated for all integral membrane proteins. ER-accumulated Golgi region proteins were functional. Photobleaching experiments indicated that Golgi-to-ER protein cycling occurred in the absence of any ER exit block. We conclude that the entire Golgi apparatus is a dynamic structure and suggest that most, if not all, Golgi region-integral membrane proteins cycle through ER in interphase cells.     

Maintenance of Golgi structure and function depends on the integrity of ER export.

The Golgi apparatus comprises an enormous array of components that generate its unique architecture and function within cells. Here, we use quantitative fluorescence imaging techniques and ultrastructural analysis to address whether the Golgi apparatus is a steady-state or a stable organelle. We found that all classes of Golgi components are dynamically associated with this organelle, contrary to the prediction of the stable organelle model. Enzymes and recycling components are continuously exiting and reentering the Golgi apparatus by membrane trafficking pathways to and from the ER, whereas Golgi matrix proteins and coatomer undergo constant, rapid exchange between membrane and cytoplasm. When ER to Golgi transport is inhibited without disrupting COPII-dependent ER export machinery (by brefeldin A treatment or expression of Arf1[T31N]), the Golgi structure disassembles, leaving no residual Golgi membranes. Rather, all Golgi components redistribute into the ER, the cytoplasm, or to ER exit sites still active for recruitment of selective membrane-bound and peripherally associated cargos. A similar phenomenon is induced by the constitutively active Sar1[H79G] mutant, which has the additional effect of causing COPII-associated membranes to cluster to a juxtanuclear region. In cells expressing Sar1[T39N], a constitutively inactive form of Sar1 that completely disrupts ER exit sites, Golgi glycosylation enzymes, matrix, and itinerant proteins all redistribute to the ER. These results argue against the hypothesis that the Golgi apparatus contains stable components that can serve as a template for its biogenesis. Instead, they suggest that the Golgi complex is a dynamic, steady-state system, whose membranes can be nucleated and are maintained by the activities of the Sar1-COPII and Arf1-coatomer systems.     

COPII under the microscope

Transport through the secretory pathway begins with COPII regulation of ER export. Driven by the Sar1 GTPase cycle, cytosolic COPII proteins exchange on and off the membrane at specific sites on the ER to regulate cargo exit. Here recent developments in COPII research are discussed, particularly the use of live-cell imaging, which has revealed surprising insights into the coat's role. The seemingly static ER exit sites are in fact highly dynamic, and the ability to visualise trafficking processes in intact living cells has highlighted the adaptable nature of COPII in cargo transport and the emerging roles of auxiliary factors.     

Concentration of Sec12 at ER exit sites via interaction with cTAGE5 is required for collagen export

Mechanisms for exporting variably sized cargo from the endoplasmic reticulum (ER) using the same machinery remain poorly understood. COPII-coated vesicles, which transport secretory proteins from the ER to the Golgi apparatus, are typically 60–90 nm in diameter. However, collagen, which forms a trimeric structure that is too large to be accommodated by conventional transport vesicles, is also known to be secreted via a COPII-dependent process. In this paper, we show that Sec12, a guanine-nucleotide exchange factor for Sar1 guanosine triphosphatase, is concentrated at ER exit sites and that this concentration of Sec12 is specifically required for the secretion of collagen VII but not other proteins. Furthermore, Sec12 recruitment to ER exit sites is organized by its direct interaction with cTAGE5, a previously characterized collagen cargo receptor component, which functions together with TANGO1 at ER exit sites. These findings suggest that the export of large cargo requires high levels of guanosine triphosphate–bound Sar1 generated by Sec12 localized at ER exit sites.     

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 YXXϕ 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.     

Trafficking, assembly, and function of a connexin43-green fluorescent protein chimera in live mammalian cells.

To examine the trafficking, assembly, and turnover of connexin43 (Cx43) in living cells, we used an enhanced red-shifted mutant of green fluorescent protein (GFP) to construct a Cx43-GFP chimera. When cDNA encoding Cx43-GFP was transfected into communication-competent normal rat kidney cells, Cx43-negative Madin-Darby canine kidney (MDCK) cells, or communication-deficient Neuro2A or HeLa cells, the fusion protein of predicted length was expressed, transported, and assembled into gap junctions that exhibited the classical pentalaminar profile. Dye transfer studies showed that Cx43-GFP formed functional gap junction channels when transfected into otherwise communication-deficient HeLa or Neuro2A cells. Live imaging of Cx43-GFP in MDCK cells revealed that many gap junction plaques remained relatively immobile, whereas others coalesced laterally within the plasma membrane. Time-lapse imaging of live MDCK cells also revealed that Cx43-GFP was transported via highly mobile transport intermediates that could be divided into two size classes of <0.5 microm and 0.5-1.5 microm. In some cases, the larger intracellular Cx43-GFP transport intermediates were observed to form from the internalization of gap junctions, whereas the smaller transport intermediates may represent other routes of trafficking to or from the plasma membrane. The localization of Cx43-GFP in two transport compartments suggests that the dynamic formation and turnover of connexins may involve at least two distinct pathways.     

Targeted gap junction protein constructs reveal connexin-specific differences in oligomerization

To define further the mechanisms of gap junction protein (connexin (Cx)) oligomerization without pharmacologic disruption, we have examined the transport and assembly of connexin constructs containing C-terminal di-lysine-based endoplasmic reticulum (ER) (HKKSL) or ER-Golgi intermediate compartment (AKKFF) targeting sequences. By immunofluorescence microscopy, Cx43-HKKSL transiently transfected into HeLa cells showed a predominantly ER localization, although Cx43-AKKFF was localized to the perinuclear region of the cell. Sucrose gradient analysis of Triton X-100-solubilized connexins showed that either Cx43-HKKSL or Cx43-AKKFF expressed alone by HeLa cells was maintained as an apparent monomer. In contrast to Cx43-HKKSL, Cx32-HKKSL was maintained in the ER as stable hexamers, consistent with the notion that Cx32 and Cx43 oligomerization occur in distinct intracellular compartments. Furthermore, Cx43-HKKSL and Cx43-AKKFF inhibited trafficking of Cx43 and Cx46 to the plasma membrane. The inhibitory effect was because of the formation of mixed oligomers between Cx43-HKKSL or Cx43-AKKF and wild type Cx43 or Cx46. Taken together, these results suggest that Cx43-HKKSL and Cx43-AKKFF recirculate through compartments where oligomerization occurs and may be maintained as apparent monomers by a putative Cx43-specific quality control mechanism.     

Scission of COPI and COPII Vesicles Is Independent of GTP Hydrolysis

Intracellular transport and maintenance of the endomembrane system in eukaryotes depends on formation and fusion of vesicular carriers. A seeming discrepancy exists in the literature about the basic mechanism in the scission of transport vesicles that depend on GTP‐binding proteins. Some reports describe that the scission of COP‐coated vesicles is dependent on GTP hydrolysis, whereas others found that GTP hydrolysis is not required. In order to investigate this pivotal mechanism in vesicle formation, we analyzed formation of COPI‐ and COPII‐coated vesicles utilizing semi‐intact cells. The small GTPases Sar1 and Arf1 together with their corresponding coat proteins, the Sec23/24 and Sec13/31 complexes for COPII and coatomer for COPI vesicles were required and sufficient to drive vesicle formation. Both types of vesicles were efficiently generated when GTP hydrolysis was blocked either by utilizing the poorly hydrolyzable GTP analogs GTPγS and GMP‐PNP, or with constitutively active mutants of the small GTPases. Thus, GTP hydrolysis is not required for the formation and release of COP vesicles.     

Melanoma progression exhibits a significant impact on connexin expression patterns in the epidermal tumor microenvironment

Melanoma depends on, interacts with and reacts to the stroma in which it is embedded, including fibroblasts, extracellular matrix, endothelial cells and immune cells. However, the impact of melanoma on the epidermal tumor microenvironment—the multilayered epithelium of the skin—is poorly understood. Gap junctions are essential for intercellular communication and involved in proliferation, differentiation and homeostasis of keratinocytes. We have shown previously that the gap junction proteins connexin 26 and 30 (Cx26 and Cx30) are induced in the epidermal tumor microenvironment of skin cancers including melanoma. This study compares the extent of Cx26, Cx30 and Cx43 expression in the epidermal microenvironment of melanocytic nevi and melanomas and its association with melanoma thickness, proliferative index of the tumor and its microenvironment, and with 5-year metastasis and survival. We found that induction of Cx26 and Cx30 cell–cell border expression in the epidermal tumor microenvironment correlates to malignancy. Importantly, there was a significant correlation of tumor thickness with the vertical epidermal Cx26 and Cx30 expression pattern and the horizontal Cx26 dissemination. Furthermore, horizontal Cx26 expression correlated with metastasis. Vertical epidermal expression patterns of Cx26 and Cx30 significantly correlated with the proliferative index in the epidermal tumor microenvironment but not with the proliferative index in the tumor. In contrast, Cx43 did not correlate with malignancy, thickness or proliferative index. In summary, here we show for the first time a significant association between the progression of melanoma and alterations in its epithelial tumor microenvironment.     

Intracellular transport, assembly, and degradation of wild-type and disease-linked mutant gap junction proteins

More than 130 different mutations in the gap junction integral plasma membrane protein connexin32 (Cx32) have been linked to the human peripheral neuropathy X-linked Charcot-Marie-Tooth disease (CMTX). How these various mutants are processed by the cell and the mechanism(s) by which they cause CMTX are unknown. To address these issues, we have studied the intracellular transport, assembly, and degradation of three CMTX-linked Cx32 mutants stably expressed in PC12 cells. Each mutant had a distinct fate: E208K Cx32 appeared to be retained in the endoplasmic reticulum (ER), whereas both the E186K and R142W mutants were transported to perinuclear compartments from which they trafficked either to lysosomes (R142W Cx32) or back to the ER (E186K Cx32). Despite these differences, each mutant was soluble in nonionic detergent but unable to assemble into homomeric connexons. Degradation of both mutant and wild-type connexins was rapid (t(1/2) < 3 h) and took place at least in part in the ER by a process sensitive to proteasome inhibitors. The mutants studied are therefore unlikely to cause disease by accumulating in degradation-resistant aggregates but instead are efficiently cleared from the cell by quality control processes that prevent abnormal connexin molecules from traversing the secretory pathway.