Recombinant Rat E3 ubiquitin-protein ligase RNF167 (Rnf167)

What is RNF167 and what is its primary cellular role?

RNF167 is a RING-type E3 ubiquitin ligase that facilitates the transfer of ubiquitin from specific E2 ubiquitin-conjugating enzymes to target substrates. This protein is primarily a transmembrane protein located in endosomes and lysosomes, where it plays a critical role in controlling the endolysosomal pathway . RNF167 is implicated in numerous cellular functions, including protein degradation via the ubiquitination pathway, protein trafficking, and lysosomal positioning . It functions as a negative regulator of lysosomal exocytosis by promoting perinuclear clustering of lysosomes, which has significant implications for cellular homeostasis .

The protein shows ubiquitous expression across human tissues, with particularly high levels in the liver, pancreas, and testis, while showing lower expression in brain tissue . Within the brain, RNF167 is expressed at similar levels in both cortex and hippocampus, highlighting its widespread distribution in neural tissues . Interestingly, while RNF167 primarily localizes to endosomes and lysosomes, studies have identified a subpopulation that resides at the plasma membrane surface of neurons, suggesting additional functions beyond the endolysosomal system .

What are the key structural features and isoforms of RNF167?

RNF167 contains a characteristic RING domain that is essential for its E3 ligase activity, enabling it to catalyze the transfer of ubiquitin to target proteins . The protein undergoes significant post-translational modifications, particularly glycosylation at two consensus sites, N33 and N79, which creates a diffuse pattern when visualized via SDS-PAGE . This glycosylation can be eliminated through treatment with endoglycosydase H (endo H) or peptide-N-glycosydase F (PNGaseF), resulting in a sharper, lower molecular mass band that indicates the removal of carbohydrate modifications . Site-directed mutagenesis experiments creating single (N33Q or N79Q) and double mutants (N33Q, N79Q) have confirmed these glycosylation sites, as each mutation produces a distinct mobility shift compared to wild-type RNF167 .

Multiple isoforms of RNF167 exist, with RNF167-a representing the full-length wild-type protein and RNF167-b lacking the first 35 amino acids that contain a putative signal peptide . These isoforms exhibit different subcellular localizations and functions, with the shorter RNF167-b showing cytoplasmic rather than lysosomal localization . In addition to these naturally occurring isoforms, several mutations in RNF167 have been identified in diverse tumor types, including the well-characterized K97N mutation, which alters protein localization and function . The molecular weight of RNF167 is approximately 61 kDa, though this can vary due to post-translational modifications and different isoforms .

What techniques are available to study RNF167's E3 ligase activity?

In vitro autoubiquitination assays represent a primary method for assessing RNF167's E3 ligase activity, allowing researchers to observe the protein's capacity to facilitate ubiquitin transfer . These assays typically involve recombinant RNF167, E1 and E2 enzymes, ubiquitin, and ATP, with ubiquitination detected via western blotting using anti-ubiquitin antibodies. For investigating interactions between RNF167 and specific E2 enzymes, binding assays can be employed to determine affinity and specificity . Fluorescence microscopy has been utilized to confirm that these interactions occur within endosomes and lysosomes, the primary locations of RNF167 .

Kinetic analyses of RNF167-E2 interactions have revealed submicromolar dissociation constants, providing quantitative measures of binding affinity . For studying the ubiquitination of specific substrates such as the AMPA receptor subunit GluA2, immunoprecipitation followed by western blotting for ubiquitin has proven effective . Research has demonstrated that in vitro polyubiquitination of GluA2 is possible through the conjugating E2 enzyme UBE2N, but only after GluA2 has been primed by ubiquitin via the action of an initiating E2 enzyme functionally binding RNF167 . Pharmacological approaches, such as inhibition of UBE2N in cultured hippocampal neurons, have been used to investigate the physiological relevance of these interactions, with results showing diminished AMPA-induced GluA2 ubiquitination following such inhibition .

How can researchers measure RNF167's impact on AMPAR trafficking?

To evaluate RNF167's effects on AMPA receptor trafficking, researchers have employed antibody-based confocal imaging assays in hippocampal neurons infected with lentiviruses expressing wild-type or mutant HA-tagged RNF167 . This approach allows for quantification of surface expression of AMPAR subunits, such as GluA2, revealing that expression of mutant RNF167 significantly increases GluA2 surface expression compared to wild-type RNF167 or uninfected cultures (133.5 ± 7.9%, 109.4 ± 4.8%, and 100.0 ± 3.9%, respectively) . Flow cytometry (FACS) analysis has also been used for quantitative assessment of surface receptor levels in larger cell populations, enabling high-throughput screening for changes in receptor expression .

For functional assessments, dual whole-cell recordings from rat organotypic hippocampal slice cultures biolistically transfected with exogenous RNF167-HA and GFP have been performed . This technique allows simultaneous comparison of evoked excitatory postsynaptic currents (EPSCs) at Schaffer collateral/CA1 synapses in transfected and nearby control cells, providing direct functional evidence of RNF167's effects . These electrophysiological studies have demonstrated that expression of mutant RNF167 significantly increases evoked AMPAR EPSC amplitude by 77%, whereas wild-type RNF167 had no effect . Importantly, NMDAR EPSCs remained unaffected by either mutant or wild-type RNF167 expression, indicating RNF167's specificity for AMPARs .

What methods can be used to study RNF167's role in lysosomal exocytosis and membrane repair?

Lysosomal exocytosis assays represent a critical approach for investigating RNF167's role in regulating this process. Researchers have employed several complementary techniques to assess how RNF167 and its variants affect lysosomal positioning and exocytosis . Immunofluorescence microscopy is commonly used to visualize lysosomal distribution, with wild-type RNF167 promoting perinuclear clustering of lysosomes, while the RNF167-K97N mutant and RNF167-b isoform fail to induce this clustering, resulting in dispersed lysosomes . This visual assessment provides important insights into how RNF167 variants affect lysosomal positioning, which directly impacts their capacity for exocytosis.

For membrane repair assessments, HeLa cells stably expressing empty vector, wild-type RNF167, or its variants have been treated with acetate Ringer's solution followed by Streptolysin-O (SLO) to induce membrane damage . Subsequent calcium-dependent resealing can then be monitored through various methods, including immunofluorescence and flow cytometry . In the flow cytometry approach, cells are stained with propidium iodide (PI) following the resealing procedure, and at least 10,000 cells are analyzed to calculate the percentage of successful plasma membrane repair . These methods have revealed that cells expressing RNF167-K97N or RNF167-b exhibit enhanced plasma membrane repair compared to those expressing wild-type RNF167, highlighting the functional consequences of these variants .

Which E2 ubiquitin-conjugating enzymes interact with RNF167 and what are their functional implications?

RNF167 functionally interacts with multiple E2 ubiquitin-conjugating enzymes, particularly UBE2D1 and UBE2N, as demonstrated through in vitro autoubiquitination and binding assays . Kinetic analyses of these interactions reveal submicromolar dissociation constants, indicating strong binding affinity between RNF167 and these E2 enzymes . These interactions primarily occur within endosomes and lysosomes, as confirmed by fluorescence microscopy studies . The specificity of these interactions is critical for determining which substrates are targeted for ubiquitination and the type of ubiquitin linkage formed, which subsequently affects the fate of the ubiquitinated proteins.

Table 1: Key E2 Enzymes Interacting with RNF167

The computed model of interaction between the RING domain of RNF167 and conjugating enzymes provides insights into the structural basis of these functional pairings . Notably, in vitro polyubiquitination of the AMPA-type glutamate receptor subunit GluA2, one of RNF167's known substrates, requires a two-step process: GluA2 must first be primed by ubiquitin through the action of an initiating E2 enzyme functionally binding RNF167, after which UBE2N can catalyze polyubiquitination . This sequential mechanism highlights the complexity of RNF167's E2 interactions and their consequences for substrate modification.

How does RNF167 regulate AMPAR-mediated synaptic transmission?

RNF167 operates as a selective regulator of AMPAR-mediated neurotransmission through its capacity to ubiquitinate AMPAR subunits, particularly GluA2, thereby controlling their surface expression and synaptic localization . AMPARs mediate the majority of fast excitatory neurotransmission in the mammalian brain, and their density at postsynaptic sites directly determines synaptic strength, making RNF167's regulatory role critically important for normal neuronal function . This regulatory mechanism has been demonstrated through experiments using a RING mutant RNF167 or specific shRNA to eliminate endogenous RNF167, which results in increased AMPAR surface expression in hippocampal neurons with disrupted RNF167 activity .

Table 2: Effects of RNF167 Manipulation on AMPAR Function

Electrophysiological studies have confirmed that RNF167 specifically regulates AMPAR currents without affecting NMDAR currents, demonstrating its selectivity for AMPAR-mediated neurotransmission . RNF167 is involved in activity-dependent ubiquitination of AMPARs, suggesting that its regulatory function is dynamically adjusted based on neuronal activity . This ubiquitination likely targets AMPARs for internalization and subsequent degradation or recycling, thereby reducing their availability at the synapse. Interestingly, while RNF167 primarily localizes to endosomes and lysosomes, a substantial fraction is surface-expressed in neurons, positioning it to directly influence surface AMPARs and possibly recruit E2 ubiquitin-conjugating enzymes involved in the ubiquitination of surface AMPARs .

What is RNF167's role in the TORC1 signaling pathway?

RNF167 serves as both a positive and negative regulator of the TORC1 signaling pathway, demonstrating context-dependent functions within this critical cellular pathway . As a positive regulator, RNF167 operates independently of arginine levels by catalyzing 'Lys-29'-polyubiquitination and degradation of CASTOR1, which releases the GATOR2 complex from CASTOR1 inhibition . This release allows GATOR2 to promote TORC1 activity, enhancing cellular anabolic processes such as protein synthesis. The specificity of this regulatory mechanism is evident in the distinct ubiquitin linkage (Lys-29) that RNF167 catalyzes on CASTOR1, highlighting the precise nature of RNF167's enzymatic activity.

Conversely, RNF167 can also negatively regulate the TORC1 signaling pathway in response to leucine deprivation, demonstrating its role in nutrient sensing and cellular adaptation . In this context, RNF167 mediates 'Lys-63'-linked polyubiquitination of SESN2, which promotes SESN2's interaction with the GATOR2 complex . This interaction inhibits GATOR2, leading to reduced TORC1 activity and downregulation of anabolic processes during nutrient limitation. The dual regulatory role of RNF167 in the TORC1 pathway, employing different ubiquitin linkages (Lys-29 versus Lys-63) on distinct substrates (CASTOR1 versus SESN2), illustrates the sophisticated molecular mechanisms through which E3 ubiquitin ligases can fine-tune cellular signaling pathways.

What mutations in RNF167 have been identified in tumors and how do they affect function?

Several naturally occurring mutations in RNF167 have been identified across diverse tumor types, with the K97N mutation being particularly well-characterized . Unlike wild-type RNF167, which predominantly localizes to lysosomes, the RNF167-K97N mutant shows cytoplasmic localization, indicating a fundamental alteration in protein trafficking or membrane association . This mislocalization has significant functional consequences, as RNF167-K97N does not promote the perinuclear lysosomal clustering that is characteristic of wild-type RNF167 activity . Instead, cells expressing RNF167-K97N exhibit dispersed lysosomes throughout the cytoplasm, altering the spatial organization of the endolysosomal system and potentially affecting numerous cellular processes dependent on proper lysosomal positioning .

The altered localization and function of RNF167-K97N result in increased lysosomal exocytosis, a process where lysosomes fuse with the plasma membrane to release their contents into the extracellular space . This enhanced exocytosis contributes to improved plasma membrane repair capabilities in cells expressing RNF167-K97N, potentially providing tumor cells with increased resistance to mechanical stress during invasion and metastasis . Interestingly, these functional features of RNF167-K97N are shared with a naturally occurring short version of RNF167, isoform b, which lacks the first 35 amino acids containing a putative signal peptide . This suggests that altered RNF167 function, whether through mutation or alternative isoform expression, may be a common mechanism contributing to tumor cell survival and progression.

How might RNF167 influence neurological conditions through its regulation of AMPA receptors?

RNF167's selective regulation of AMPAR-mediated neurotransmission positions it as a potential contributor to various neurological conditions where glutamatergic signaling is dysregulated . AMPARs mediate the majority of fast excitatory neurotransmission in the mammalian brain, and alterations in excitatory synaptic strength are believed to be the cellular basis for learning and memory . By controlling AMPAR surface expression through ubiquitination, RNF167 can directly influence synaptic strength and plasticity, which are fundamental to cognitive processes. Disruptions in RNF167 function could therefore potentially impact learning, memory, and other cognitive domains, suggesting relevance to conditions such as intellectual disability, autism spectrum disorders, or neurodegenerative diseases.

Experimental evidence demonstrates that manipulation of RNF167 activity through mutation or knockdown significantly increases AMPAR surface expression and synaptic currents, highlighting its importance for maintaining appropriate excitatory balance . Since excitatory/inhibitory imbalance is implicated in various neuropsychiatric conditions, including epilepsy, autism, and schizophrenia, RNF167 dysfunction might contribute to the pathophysiology of these disorders. Furthermore, the specificity of RNF167 for AMPARs but not NMDARs suggests it could be targeted to selectively modulate specific aspects of glutamatergic transmission without broadly affecting all excitatory signaling . This selectivity could be advantageous for potential therapeutic interventions aimed at normalizing excitatory transmission in conditions where AMPAR-mediated signaling is specifically altered.

What is the potential connection between RNF167, lysosomal exocytosis, and cancer progression?

The discovery that tumor-associated RNF167 variants (both the K97N mutant and isoform b) enhance lysosomal exocytosis and plasma membrane repair establishes a potential mechanistic link between RNF167 and cancer progression . Lysosomal exocytosis plays important roles in multiple aspects of tumor biology, including extracellular matrix degradation, cell migration, and resistance to membrane damage during invasion through tissues . By negatively regulating lysosomal exocytosis under normal conditions, wild-type RNF167 may function as a brake on these potentially pro-tumorigenic processes. Conversely, mutations or isoform switching that compromise this regulatory function could remove this brake, facilitating more aggressive cancer cell behaviors.

Enhanced plasma membrane repair capability conferred by RNF167 variants provides cancer cells with increased resilience against mechanical stress encountered during invasion and metastasis . This repair mechanism involves calcium-dependent fusion of lysosomes with the plasma membrane, a process that would be facilitated by the dispersed lysosomal distribution observed in cells expressing RNF167-K97N or RNF167-b . Additionally, RNF167's regulation of ARL8B, which controls lysosome positioning, provides another potential mechanism linking RNF167 to cancer progression . Wild-type RNF167 catalyzes ubiquitination and degradation of ARL8B, while tumor-associated variants may lack this ability, leading to dysregulated ARL8B levels and abnormal lysosomal distribution . This multifaceted involvement of RNF167 in processes relevant to cancer progression suggests it could serve as both a biomarker and potential therapeutic target in oncology.