SHOC2 subcellular shuttling requires the KEKE motif-rich region and N-terminal leucine-rich repeat domain and impacts on ERK signalling

The expression of congenital Shoc2 variants induces AKT-dependent crosstalk activation of the ERK1/2 pathway

The Shoc2 scaffold protein is crucial in transmitting signals within the Epidermal Growth Factor Receptor (EGFR)-mediated Extracellular signal-Regulated Kinase (ERK1/2) pathway. While the significance of Shoc2 in this pathway is well-established, the precise mechanisms through which Shoc2 governs signal transmission remain to be fully elucidated. Hereditary variants in Shoc2 are responsible for Noonan Syndrome with Loose anagen Hair (NSLH). However, due to the absence of known enzymatic activity in Shoc2, directly assessing how these variants affect its function is challenging. ERK1/2 phosphorylation is used as a primary parameter of Shoc2 function, but the impact of Shoc2 mutants on the pathway activation is unclear. This study investigates how the NSLH-associated Shoc2 variants influence EGFR signals in the context of the ERK1/2 and AKT downstream signaling pathways. We show that when the ERK1/2 pathway is a primary signaling pathway activated downstream of EGFR, Shoc2 variants cannot upregulate ERK1/2 phosphorylation to the level of the WT Shoc2. Yet, when the AKT and ERK1/2 pathways were activated, in cells expressing Shoc2 variants, ERK1/2 phosphorylation was higher than in cells expressing WT Shoc2. In cells expressing the Shoc2 NSLH mutants, we found that the AKT signaling pathway triggers the PAK activation, followed by phosphorylation of Raf-1/MEK1/2 and activation of the ERK1/2 signaling axis. Hence, our studies reveal a previously unrecognized feedback regulation downstream of the EGFR and provide additional evidence for the role of Shoc2 as a "gatekeeper" in controlling the selection of downstream effectors within the EGFR signaling network.

The expression of congenital Shoc2 variants induces AKT-dependent feedback activation of the ERK1/2 pathway

The Shoc2 scaffold protein is crucial in transmitting signals within the Epidermal Growth Factor Receptor (EGFR)-mediated Extracellular signal-regulated Kinase (ERK1/2) pathway. While the significance of Shoc2 in this pathway is well-established, the precise mechanisms through which Shoc2 governs signal transmission remain to be fully elucidated. Hereditary mutations in Shoc2 are responsible for Noonan Syndrome with Loose anagen Hair (NSLH). However, due to the absence of known enzymatic activity in Shoc2, directly assessing how these mutations affect its function is challenging. ERK1/2 phosphorylation is used as a primary parameter of Shoc2 function, but the impact of Shoc2 mutants on the pathway activation is unclear. This study investigates how the NSLH-associated Shoc2 variants influence EGFR signals in the context of the ERK1/2 and AKT downstream signaling pathways. We show that when the ERK1/2 pathway is a primary signaling pathway activated downstream of EGFR, Shoc2 variants cannot upregulate ERK1/2 phosphorylation to the level of the WT Shoc2. Yet, when the AKT and ERK1/2 pathways were activated, in cells expressing Shoc2 variants, ERK1/2 phosphorylation was higher than in cells expressing WT Shoc2. We found that, in cells expressing the Shoc2 NSLH mutants, the AKT signaling pathway triggers the PAK activation, followed by phosphorylation and Raf-1/MEK1/2 /ERK1/2 signaling axis activation. Hence, our studies reveal a previously unrecognized feedback regulation downstream of the EGFR and provide evidence for the Shoc2 role as a "gatekeeper" in controlling the selection of downstream effectors within the EGFR signaling network.

Structural insights into the role of SHOC2-MRAS-PP1C complex in RAF activation

RAF activation is a key step for signalling through the mitogen-activated protein kinase (MAPK) pathway. The SHOC2 protein, along with MRAS and PP1C, forms a high affinity, heterotrimeric holoenzyme that activates RAF kinases by dephosphorylating a specific phosphoserine. Recently, our research, along with that of three other teams, has uncovered valuable structural and functional insights into the SHOC2-MRAS-PP1C (SMP) holoenzyme complex. In this structural snapshot, we review SMP complex assembly, the dependency on the bound-nucleotide state of MRAS, the substitution of MRAS by the canonical RAS proteins and the roles of SHOC2 and MRAS on PP1C activity and specificity. Furthermore, we discuss the effect of several RASopathy mutations identified within the SMP complex and explore potential therapeutic approaches for targeting the SMP complex in RAS/RAF-driven cancers and RASopathies.

Structure-function analysis of the SHOC2-MRAS-PP1C holophosphatase complex

Receptor tyrosine kinase (RTK)-RAS signalling through the downstream mitogen-activated protein kinase (MAPK) cascade regulates cell proliferation and survival. The SHOC2-MRAS-PP1C holophosphatase complex functions as a key regulator of RTK-RAS signalling by removing an inhibitory phosphorylation event on the RAF family of proteins to potentiate MAPK signalling1. SHOC2 forms a ternary complex with MRAS and PP1C, and human germline gain-of-function mutations in this complex result in congenital RASopathy syndromes2-5. However, the structure and assembly of this complex are poorly understood. Here we use cryo-electron microscopy to resolve the structure of the SHOC2-MRAS-PP1C complex. We define the biophysical principles of holoenzyme interactions, elucidate the assembly order of the complex, and systematically interrogate the functional consequence of nearly all of the possible missense variants of SHOC2 through deep mutational scanning. We show that SHOC2 binds PP1C and MRAS through the concave surface of the leucine-rich repeat region and further engages PP1C through the N-terminal disordered region that contains a cryptic RVXF motif. Complex formation is initially mediated by interactions between SHOC2 and PP1C and is stabilized by the binding of GTP-loaded MRAS. These observations explain how mutant versions of SHOC2 in RASopathies and cancer stabilize the interactions of complex members to enhance holophosphatase activity. Together, this integrative structure-function model comprehensively defines key binding interactions within the SHOC2-MRAS-PP1C holophosphatase complex and will inform therapeutic development .

Structural basis for SHOC2 modulation of RAS signalling

The RAS-RAF pathway is one of the most commonly dysregulated in human cancers1-3. Despite decades of study, understanding of the molecular mechanisms underlying dimerization and activation4 of the kinase RAF remains limited. Recent structures of inactive RAF monomer5 and active RAF dimer5-8 bound to 14-3-39,10 have revealed the mechanisms by which 14-3-3 stabilizes both RAF conformations via specific phosphoserine residues. Prior to RAF dimerization, the protein phosphatase 1 catalytic subunit (PP1C) must dephosphorylate the N-terminal phosphoserine (NTpS) of RAF11 to relieve inhibition by 14-3-3, although PP1C in isolation lacks intrinsic substrate selectivity. SHOC2 is as an essential scaffolding protein that engages both PP1C and RAS to dephosphorylate RAF NTpS11-13, but the structure of SHOC2 and the architecture of the presumptive SHOC2-PP1C-RAS complex remain unknown. Here we present a cryo-electron microscopy structure of the SHOC2-PP1C-MRAS complex to an overall resolution of 3 Å, revealing a tripartite molecular architecture in which a crescent-shaped SHOC2 acts as a cradle and brings together PP1C and MRAS. Our work demonstrates the GTP dependence of multiple RAS isoforms for complex formation, delineates the RAS-isoform preference for complex assembly, and uncovers how the SHOC2 scaffold and RAS collectively drive specificity of PP1C for RAF NTpS. Our data indicate that disease-relevant mutations affect complex assembly, reveal the simultaneous requirement of two RAS molecules for RAF activation, and establish rational avenues for discovery of new classes of inhibitors to target this pathway.

Expanding the molecular spectrum of pathogenic SHOC2 variants underlying Mazzanti syndrome

We previously molecularly and clinically characterized Mazzanti syndrome, a RASopathy related to Noonan syndrome that is mostly caused by a single recurrent missense variant (c.4A > G, p.Ser2Gly) in SHOC2, which encodes a leucine-rich repeat (LRR)-containing protein facilitating signal flow through the RAS-mitogen-associated protein kinase (MAPK) pathway. We also documented that the pathogenic p.Ser2Gly substitution causes upregulation of MAPK signaling and constitutive targeting of SHOC2 to the plasma membrane due to the introduction of an N-myristoylation recognition motif. The almost invariant occurrence of the pathogenic c.4A > G missense change in SHOC2 is mirrored by a relatively homogeneous clinical phenotype of Mazzanti syndrome. Here we provide new data on the clinical spectrum and molecular diversity of this disorder, and functionally characterize new pathogenic variants. The clinical phenotype of six unrelated individuals carrying novel disease-causing SHOC2 variants is delineated, and public and newly collected clinical data are utilized to profile the disorder. In silico, in vitro and in vivo characterization of the newly identified variants provides evidence that the consequences of these missense changes on SHOC2 functional behavior differ from what had been observed for the canonical p.Ser2Gly change but converge towards an enhanced activation of the RAS-MAPK pathway. Our findings expand the molecular spectrum of pathogenic SHOC2 variants, provide a more accurate picture of the phenotypic expression associated with variants in this gene, and definitively establish a GoF behavior as the mechanism of disease.

SPRED2 loss-of-function causes a recessive Noonan syndrome-like phenotype

Upregulated signal flow through RAS and the mitogen-associated protein kinase (MAPK) cascade is the unifying mechanistic theme of the RASopathies, a family of disorders affecting development and growth. Pathogenic variants in more than 20 genes have been causally linked to RASopathies, the majority having a dominant role in promoting enhanced signaling. Here, we report that SPRED2 loss of function is causally linked to a recessive phenotype evocative of Noonan syndrome. Homozygosity for three different variants-c.187C>T (p.Arg63^∗), c.299T>C (p.Leu100Pro), and c.1142_1143delTT (p.Leu381Hisfs^∗95)-were identified in four subjects from three families. All variants severely affected protein stability, causing accelerated degradation, and variably perturbed SPRED2 functional behavior. When overexpressed in cells, all variants were unable to negatively modulate EGF-promoted RAF1, MEK, and ERK phosphorylation, and time-course experiments in primary fibroblasts (p.Leu100Pro and p.Leu381Hisfs^∗95) documented an increased and prolonged activation of the MAPK cascade in response to EGF stimulation. Morpholino-mediated knockdown of spred2a and spred2b in zebrafish induced defects in convergence and extension cell movements indicating upregulated RAS-MAPK signaling, which were rescued by expressing wild-type SPRED2 but not the SPRED2^{Leu381Hisfs∗95} protein. The clinical phenotype of the four affected individuals included developmental delay, intellectual disability, cardiac defects, short stature, skeletal anomalies, and a typical facial gestalt as major features, without the occurrence of the distinctive skin signs characterizing Legius syndrome. These features, in part, characterize the phenotype of Spred2^-/- mice. Our findings identify the second recessive form of Noonan syndrome and document pleiotropic consequences of SPRED2 loss of function in development.

Activating MRAS mutations cause Noonan syndrome associated with hypertrophic cardiomyopathy

The RASopathies are a group of genetic syndromes caused by upregulated RAS signaling. Noonan syndrome (NS), the most common entity among the RASopathies, is characterized mainly by short stature, cardiac anomalies and distinctive facial features. Mutations in multiple RAS-MAPK pathway-related genes have been associated with NS and related phenotypes. We describe two unrelated patients presenting with hypertrophic cardiomyopathy (HCM) and dysmorphic features suggestive of NS. One of them died in the neonatal period because of cardiac failure. Targeted sequencing revealed de novo MRAS variants, c.203C > T (p.Thr68Ile) and c.67G > C (p.Gly23Arg) as causative events. MRAS has only recently been related to NS based on the observation of two unrelated affected individuals with de novo variants involving the same codons here found mutated. Gly23 and Thr68 are highly conserved residues, and the corresponding codons are known hotspots for RASopathy-associated mutations in other RAS proteins. Functional analyses documented high level of activation of MRAS mutants due to impaired GTPase activity, which was associated with constitutive plasma membrane targeting, prolonged localization in non-raft microdomains, enhanced binding to PPP1CB and SHOC2 protein, and variably increased MAPK and PI3K-AKT activation. This report provides additional evidence that a narrow spectrum of activating mutations in MRAS represents another rare cause of NS, and that MRAS has to be counted among the RASopathy genes predisposing to HCM. Moreover, our findings further emphasize the relevance of the MRAS-SHOC2-PPP1CB axis in the control of MAPK signaling, and the contribution of both MAPK and PI3K-AKT pathways in MRAS functional upregulation.

Activin-A Induces Early Differential Gene Expression Exclusively in Periodontal Ligament Fibroblasts from Fibrodysplasia Ossificans Progressiva Patients

Fibrodysplasia Ossificans Progressiva (FOP) is a rare genetic disease characterized by heterotopic ossification (HO). It is caused by mutations in the Activin receptor type 1 (ACVR1) gene, resulting in enhanced responsiveness to ligands, specifically to Activin-A. Though it has been shown that capturing Activin-A protects against heterotopic ossification in animal models, the exact underlying mechanisms at the gene expression level causing ACVR1 R206H-mediated ossifications and progression are thus far unknown. We investigated the early transcriptomic changes induced by Activin-A of healthy control and patient-derived periodontal ligament fibroblasts (PLF) isolated from extracted teeth by RNA sequencing analysis. To study early differences in response to Activin-A, periodontal ligament fibroblasts from six control teeth and from six FOP patient teeth were cultured for 24 h without and with 50 ng/mL Activin-A and analyzed with RNA sequencing. Pathway analysis on genes upregulated by Activin-A in FOP cells showed an association with pathways involved in, among others, Activin, TGFβ, and BMP signaling. Differential gene expression induced by Activin-A was exclusively seen in the FOP cells. Median centered supervised gene expression analysis showed distinct clusters of up- and downregulated genes in the FOP cultures after stimulation with Activin-A. The upregulated genes with high fold changes like SHOC2, TTC1, PAPSS2, DOCK7, and LOX are all associated with bone metabolism. Our open-ended approach to investigating the early effect of Activin-A on gene expression in control and FOP PLF shows that the molecule exclusively induces differential gene expression in FOP cells and not in control cells.

A Leucine-Rich Repeat Protein Provides a SHOC2 the RAS Circuit: a Structure-Function Perspective

SHOC2 is a prototypical leucine-rich repeat protein that promotes downstream receptor tyrosine kinase (RTK)/RAS signaling and plays important roles in several cellular and developmental processes. Gain-of-function germ line mutations of SHOC2 drive the RASopathy Noonan-like syndrome, and SHOC2 mediates adaptive resistance to mitogen-activated protein kinase (MAPK) inhibitors. Similar to many scaffolding proteins, SHOC2 facilitates signal transduction by enabling proximal protein interactions and regulating the subcellular localization of its binding partners. Here, we review the structural features of SHOC2 that mediate its known functions, discuss these elements in the context of various binding partners and signaling pathways, and highlight areas of SHOC2 biology where a consensus view has not yet emerged.

SHOC2 Is a Critical Modulator of Sensitivity to EGFR-TKIs in Non-Small Cell Lung Cancer Cells

EGFR mutation-positive patients with non-small cell lung cancer (NSCLC) respond well to treatment with EGFR-tyrosine kinase inhibitors (EGFR-TKI); however, treatment with EGFR-TKIs is not curative, owing to the presence of residual cancer cells with intrinsic or acquired resistance to this class of drugs. Additional treatment targets that may enhance the efficacy of EGFR-TKIs remain elusive. Using a CRISPR/Cas9-based screen, we identified the leucine-rich repeat scaffold protein SHOC2 as a key modulator of sensitivity to EGFR-TKI treatment. On the basis of in vitro assays, we demonstrated that SHOC2 expression levels strongly correlate with the sensitivity to EGFR-TKIs and that SHOC2 affects the sensitivity to EGFR-TKIs in NSCLC cells via SHOC2/MRAS/PP1c and SHOC2/SCRIB signaling. The potential SHOC2 inhibitor celastrol phenocopied SHOC2 depletion. In addition, we confirmed that SHOC2 expression levels were important for the sensitivity to EGFR-TKIs in vivo. Furthermore, IHC showed the accumulation of cancer cells that express high levels of SHOC2 in lung cancer tissues obtained from patients with NSCLC who experienced acquired resistance to EGFR-TKIs. These data indicate that SHOC2 may be a therapeutic target for patients with NSCLC or a biomarker to predict sensitivity to EGFR-TKI therapy in EGFR mutation-positive patients with NSCLC. Our findings may help improve treatment strategies for patients with NSCLC harboring EGFR mutations. IMPLICATIONS: This study showed that SHOC2 works as a modulator of sensitivity to EGFR-TKIs and the expression levels of SHOC2 can be used as a biomarker for sensitivity to EGFR-TKIs.

Enhanced MAPK1 Function Causes a Neurodevelopmental Disorder within the RASopathy Clinical Spectrum

Signal transduction through the RAF-MEK-ERK pathway, the first described mitogen-associated protein kinase (MAPK) cascade, mediates multiple cellular processes and participates in early and late developmental programs. Aberrant signaling through this cascade contributes to oncogenesis and underlies the RASopathies, a family of cancer-prone disorders. Here, we report that de novo missense variants in MAPK1, encoding the mitogen-activated protein kinase 1 (i.e., extracellular signal-regulated protein kinase 2, ERK2), cause a neurodevelopmental disease within the RASopathy phenotypic spectrum, reminiscent of Noonan syndrome in some subjects. Pathogenic variants promote increased phosphorylation of the kinase, which enhances translocation to the nucleus and boosts MAPK signaling in vitro and in vivo. Two variant classes are identified, one of which directly disrupts binding to MKP3, a dual-specificity protein phosphatase negatively regulating ERK function. Importantly, signal dysregulation driven by pathogenic MAPK1 variants is stimulus reliant and retains dependence on MEK activity. Our data support a model in which the identified pathogenic variants operate with counteracting effects on MAPK1 function by differentially impacting the ability of the kinase to interact with regulators and substrates, which likely explains the minor role of these variants as driver events contributing to oncogenesis. After nearly 20 years from the discovery of the first gene implicated in Noonan syndrome, PTPN11, the last tier of the MAPK cascade joins the group of genes mutated in RASopathies.

Dominant Noonan syndrome-causing LZTR1 mutations specifically affect the Kelch domain substrate-recognition surface and enhance RAS-MAPK signaling

Noonan syndrome (NS), the most common RASopathy, is caused by mutations affecting signaling through RAS and the MAPK cascade. Recently, genome scanning has discovered novel genes implicated in NS, whose function in RAS-MAPK signaling remains obscure, suggesting the existence of unrecognized circuits contributing to signal modulation in this pathway. Among these genes, leucine zipper-like transcriptional regulator 1 (LZTR1) encodes a functionally poorly characterized member of the BTB/POZ protein superfamily. Two classes of germline LZTR1 mutations underlie dominant and recessive forms of NS, while constitutional monoallelic, mostly inactivating, mutations in the same gene cause schwannomatosis, a cancer-prone disorder clinically distinct from NS. Here we show that dominant NS-causing LZTR1 mutations do not affect significantly protein stability and subcellular localization. We provide the first evidence that these mutations, but not the missense changes occurring as biallelic mutations in recessive NS, enhance stimulus-dependent RAS-MAPK signaling, which is triggered, at least in part, by an increased RAS protein pool. Moreover, we document that dominant NS-causing mutations do not perturb binding of LZTR1 to CUL3, a scaffold coordinating the assembly of a multimeric complex catalyzing protein ubiquitination but are predicted to affect the surface of the Kelch domain mediating substrate binding to the complex. Collectively, our data suggest a model in which LZTR1 contributes to the ubiquitinationof protein(s) functioning as positive modulator(s) of the RAS-MAPK signaling pathway. In this model, LZTR1 mutations are predicted to variably impair binding of these substrates to the multi-component ligase complex and their efficient ubiquitination and degradation, resulting in MAPK signaling upregulation.

Clinical and functional characterization of a novel RASopathy-causing SHOC2 mutation associated with prenatal-onset hypertrophic cardiomyopathy

SHOC2 is a scaffold protein mediating RAS-promoted activation of mitogen-activated protein kinase (MAPK) signaling in response to extracellular stimuli. A recurrent activating mutation in SHOC2 (p.Ser2Gly) causes Mazzanti syndrome, a RASopathy characterized by features resembling Noonan syndrome and distinctive ectodermal abnormalities. A second mutation (p.Met173Ile) supposed to cause loss-of-function was more recently identified in two individuals with milder phenotypes. Here, we report on the third RASopathy-causing SHOC2 mutation (c.807_808delinsTT, p.Gln269_His270delinsHisTyr), which was found associated with prenatal-onset hypertrophic cardiomyopathy. Structural analyses indicated a possible impact of the mutation on the relative orientation of the two SHOC2's leucine-rich repeat domains. Functional studies provided evidence of its activating role, revealing enhanced binding of the mutant protein to MRAS and PPP1CB, and increased signaling through the MAPK cascade. Differing from SHOC2 ^S2G , SHOC2 ^{Q269_H270delinsHY} is not constitutively targeted to the plasma membrane. These data document that diverse mechanisms in SHOC2 functional dysregulation converge toward MAPK signaling upregulation.

SHOC2-MRAS-PP1 complex positively regulates RAF activity and contributes to Noonan syndrome pathogenesis

Dephosphorylation of the inhibitory "S259" site on RAF kinases (S259 on CRAF, S365 on BRAF) plays a key role in RAF activation. The MRAS GTPase, a close relative of RAS oncoproteins, interacts with SHOC2 and protein phosphatase 1 (PP1) to form a heterotrimeric holoenzyme that dephosphorylates this S259 RAF site. MRAS and SHOC2 function as PP1 regulatory subunits providing the complex with striking specificity against RAF. MRAS also functions as a targeting subunit as membrane localization is required for efficient RAF dephosphorylation and ERK pathway regulation in cells. SHOC2's predicted structure shows remarkable similarities to the A subunit of PP2A, suggesting a case of convergent structural evolution with the PP2A heterotrimer. We have identified multiple regions in SHOC2 involved in complex formation as well as residues in MRAS switch I and the interswitch region that help account for MRAS's unique effector specificity for SHOC2-PP1. MRAS, SHOC2, and PPP1CB are mutated in Noonan syndrome, and we show that syndromic mutations invariably promote complex formation with each other, but not necessarily with other interactors. Thus, Noonan syndrome in individuals with SHOC2, MRAS, or PPPC1B mutations is likely driven at the biochemical level by enhanced ternary complex formation and highlights the crucial role of this phosphatase holoenzyme in RAF S259 dephosphorylation, ERK pathway dynamics, and normal human development.

The RASopathy Family: Consequences of Germline Activation of the RAS/MAPK Pathway

Noonan syndrome [NS; Mendelian Inheritance in Men (MIM) #163950] and related syndromes [Noonan syndrome with multiple lentigines (formerly called LEOPARD syndrome; MIM #151100), Noonan-like syndrome with loose anagen hair (MIM #607721), Costello syndrome (MIM #218040), cardio-facio-cutaneous syndrome (MIM #115150), type I neurofibromatosis (MIM #162200), and Legius syndrome (MIM #611431)] are a group of related genetic disorders associated with distinctive facial features, cardiopathies, growth and skeletal abnormalities, developmental delay/mental retardation, and tumor predisposition. NS was clinically described more than 50 years ago, and disease genes have been identified throughout the last 3 decades, providing a molecular basis to better understand their physiopathology and identify targets for therapeutic strategies. Most of these genes encode proteins belonging to or regulating the so-called RAS/MAPK signaling pathway, so these syndromes have been gathered under the name RASopathies. In this review, we provide a clinical overview of RASopathies and an update on their genetics. We then focus on the functional and pathophysiological effects of RASopathy-causing mutations and discuss therapeutic perspectives and future directions.

Exploring the autoinhibitory domain of the electrogenic Na+ /HCO3- transporter NBCe1-B, from residues 28 to 62

KEY POINTS

Slc4a4 (mouse) encodes at least five variants of the electrogenic sodium/bicarbonate transporter NBCe1. The initial 41 cytosolic amino acids of NBCe1-A and -D are unique; NBCe1-A has high activity. The initial 85 amino acids of NBCe1-B, -C and -E are unique; NBCe1-B and -C have low activity. Previous work showed that deleting residues 1-85 or 40-62 of NBCe1-B, or 1-87 of NBCe1-C, eliminates autoinhibition. These regions also include binding determinants for IRBIT (inositol trisphosphate (IP₃ )-receptor binding protein released with IP₃ ), which relieves autoinhibition. Here, systematically replacing/deleting residues 28-62, we find that only the nine amino acid cationic cluster (residues 40-48) of NBCe1-B is essential for autoinhibition. IRBIT stimulates all but one low-activity construct. We suggest that electrostatic interactions - which IRBIT presumably interrupts - between the cationic cluster and the membrane or other domains of NBCe1 play a central role in tempering the activity of NBCe1-B in the pancreas, brain and other organs.

ABSTRACT

Variant B of the electrogenic Na⁺ /HCO₃^- cotransporter (NBCe1-B) contributes to the vectorial transport of HCO₃^- in epithelia (e.g. pancreatic ducts) and to the maintenance of intracellular pH in the central nervous systems (e.g. astrocytes). NBCe1-B has very low basal activity due to an autoinhibitory domain (AID) located, at least in part, in the unique portion (residues 1-85) of the cytosolic NH₂ -terminus. Previous work has shown that removing 23 amino acids (residues 40-62) stimulates NBCe1-B. Here, we test the hypothesis that a cationic cluster of nine consecutive positively charged amino acids (residues 40-48) is a necessary part of the AID. Using two-electrode voltage clamping of Xenopus oocytes, we assess the activity of human NBCe1-B constructs in which we systematically replace or delete residues 28-62, which includes the cationic cluster. We find that replacing or deleting all residues within the cationic cluster markedly increases NBCe1-B activity (i.e. eliminates autoinhibition). On the background of a cationic clusterless construct, systematically restoring Arg residues restores autoinhibition in two distinct quanta, with one to three Arg residues restoring ∼50%, and four or more Arg residues restoring virtually all autoinhibition. Systematically deleting residues before the cluster reduces autoinhibition by, at most, a small amount. Replacing or deleting residues after the cluster has no effect. For constructs with low NBCe1 activity (but good surface expression, as assessed by biotinylation), co-expression with super-IRBIT (lacking PP1-binding site) restores full activity (i.e. relieves autoinhibition). In summary, the cationic cluster is a necessary component of the AID of NBCe1-B.