The PDZ1 and PDZ3 domains of MAGI-1 regulate the eight-exon isoform of the coxsackievirus and adenovirus receptor

CXADR: From an Essential Structural Component to a Vital Signaling Mediator in Spermatogenesis

Canonical coxsackievirus and adenovirus receptor (CXADR) is a transmembrane component of cell junctions that is crucial for cardiac and testicular functions via its homophilic and heterophilic interaction. CXADR is expressed in both Sertoli cells and germ cells and is localized mainly at the interface between Sertoli-Sertoli cells and Sertoli-germ cells. Knockout of CXADR in mouse Sertoli cells specifically impairs male reproductive functions, including a compromised blood-testis barrier, apoptosis of germ cells, and premature loss of spermatids. Apart from serving as an important component for cell junctions, recent progress has showed the potential roles of CXADR as a signaling mediator in spermatogenesis. This review summarizes current research progress related to the regulation and role of CXADR in spermatogenesis as well as in pathological conditions. We hope this review provides some future directions and a blueprint to promote the further study on the roles of CXADR.

The IgCAM CAR Regulates Gap Junction-Mediated Coupling on Embryonic Cardiomyocytes and Affects Their Beating Frequency

The IgCAM coxsackie-adenovirus receptor (CAR) is essential for embryonic heart development and electrical conduction in the mature heart. However, it is not well-understood how CAR exerts these effects at the cellular level. To address this question, we analyzed the spontaneous beating of cultured embryonic hearts and cardiomyocytes from wild type and CAR knockout (KO) embryos. Surprisingly, in the absence of the CAR, cultured cardiomyocytes showed increased frequencies of beating and calcium cycling. Increased beatings of heart organ cultures were also induced by the application of reagents that bind to the extracellular region of the CAR, such as the adenovirus fiber knob. However, the calcium cycling machinery, including calcium extrusion via SERCA2 and NCX, was not disrupted in CAR KO cells. In contrast, CAR KO cardiomyocytes displayed size increases but decreased in the total numbers of membrane-localized Cx43 clusters. This was accompanied by improved cell-cell coupling between CAR KO cells, as demonstrated by increased intercellular dye diffusion. Our data indicate that the CAR may modulate the localization and oligomerization of Cx43 at the plasma membrane, which could in turn influence electrical propagation between cardiomyocytes via gap junctions.

The magic of MAGI-1: A scaffolding protein with multi signalosomes and functional plasticity

MAGI-1 is a critical cellular scaffolding protein with over 110 different cellular and microbial protein interactors. Since the discovery of MAGI-1 in 1997, MAGI-1 has been implicated in diverse cellular functions such as polarity, cell-cell communication, neurological processes, kidney function, and a host of diseases including cancer and microbial infection. Additionally, MAGI-1 has undergone nomenclature changes in response to the discovery of an additional PDZ domain, leading to lack of continuity in the literature. We address the nomenclature of MAGI-1 as well as summarize many of the critical functions of the known interactions. Given the importance of many of the interactors, such as human papillomavirus E6, the Coxsackievirus and adenovirus receptor (CAR), and PTEN, the enhancement or disruption of MAGI-based interactions has the potential to affect cellular functions that can potentially be harnessed as a therapeutic strategy for a variety of diseases.

The Coxsackievirus and Adenovirus Receptor Has a Short Half-Life in Epithelial Cells

The coxsackievirus and adenovirus receptor (CAR) is an essential cellular protein that is involved in cell adhesion, cell signaling, and viral infection. The 8-exon encoded isoform (CAREx8) resides at the apical surface of polarized epithelia, where it is accessible as a receptor for adenovirus entering the airway lumen. Given its pivotal role in viral infection, it is a target for antiviral strategies. To understand the regulation of CAREx8 and determine the feasibility of receptor downregulation, the half-life of total and apical localized CAREx8 was determined and correlated with adenovirus transduction. Total and apical CAREx8 has a relatively short half-life of approximately 2 h. The half-life of apical CAREx8 correlates well with adenovirus transduction. These results suggest that antiviral strategies that aim to degrade the primary receptor for apical adenovirus infection will be effective within a relatively short time frame after application.

MAGI-1 PDZ2 Domain Blockade Averts Adenovirus Infection via Enhanced Proteolysis of the Apical Coxsackievirus and Adenovirus Receptor

Adenoviruses (AdVs) are etiological agents of gastrointestinal, heart, eye, and respiratory tract infections that can be lethal for immunosuppressed people. Many AdVs use the coxsackievirus and adenovirus receptor (CAR) as a primary receptor. The CAR isoform resulting from alternative splicing that includes the eighth exon, CAR^Ex8, localizes to the apical surface of polarized epithelial cells and is responsible for the initiation of AdV infection. We have shown that the membrane level of CAR^Ex8 is tightly regulated by two MAGI-1 PDZ domains, PDZ2 and PDZ4, resulting in increased or decreased AdV transduction, respectively. We hypothesized that targeting the interactions between the MAGI-1 PDZ2 domain and CAR^Ex8 would decrease the apical CAR^Ex8 expression level and prevent AdV infection. Decoy peptides that target MAGI-1 PDZ2 were synthesized (TAT-E6 and TAT-NET1). PDZ2 binding peptides decreased CAR^Ex8 expression and reduced AdV transduction. CAR^Ex8 degradation was triggered by the activation of the regulated intramembrane proteolysis (RIP) pathway through a disintegrin and metalloproteinase (ADAM17) and γ-secretase. Further analysis revealed that ADAM17 interacts directly with the MAGI-1 PDZ3 domain, and blocking the PDZ2 domain enhanced the accessibility of ADAM17 to the substrate (CAR^Ex8). Finally, we validated the efficacy of TAT-PDZ2 peptides in protecting the epithelia from AdV transduction in vivo using a novel transgenic animal model. Our data suggest that TAT-PDZ2 binding peptides are novel anti-AdV molecules that act by enhanced RIP of CAR^Ex8 and decreased AdV entry. This strategy has additional translational potential for targeting other viral receptors that have PDZ binding domains, such as the angiotensin-converting enzyme 2 receptor. IMPORTANCE Adenovirus is a common threat in immunosuppressed populations and military recruits. There are no currently approved treatments/prophylactic agents that protect from most AdV infections. Here, we developed peptide-based small molecules that can suppress AdV infection of polarized epithelia by targeting the AdV receptor, coxsackievirus and adenovirus receptor (CAR^Ex8). The newly discovered peptides target a specific PDZ domain of the CAR^Ex8-interacting protein MAGI-1 and decrease AdV transduction in multiple polarized epithelial models. Peptide-induced CAR^Ex8 degradation is triggered by extracellular domain (ECD) shedding through ADAM17 followed by γ-secretase-mediated nuclear translocation of the C-terminal domain. The enhanced shedding of the CAR^Ex8 ECD further protected the epithelium from AdV infection. Taken together, these novel molecules protect the epithelium from AdV infection. This approach may be applicable to the development of novel antiviral molecules against other viruses that use a receptor with a PDZ binding domain.

QTLs Associated with Resistance to Cardiomyopathy Syndrome in Atlantic Salmon

Cardiomyopathy syndrome (CMS) caused by piscine myocarditis virus is a major disease affecting the Norwegian Atlantic salmon industry. Three different populations of Atlantic salmon from the Mowi breeding program were used in this study. The first 2 populations (population 1 and 2) were naturally infected in a field outbreak, while the third population (population 3) went through a controlled challenged test. The aim of the study was to estimate the heritability, the genetic correlation between populations and perform genome-wide association analysis for resistance to this disease. Survival data from population 1 and 2 and heart atrium histology score data from population 3 was analyzed. A total of 571, 4312, and 901 fish from population 1, 2, and 3, respectively were genotyped with a noncommercial 55,735 Affymetrix marker panel. Genomic heritability ranged from 0.12 to 0.46 and the highest estimate was obtained from the challenge test dataset. The genetic correlation between populations was moderate (0.51–0.61). Two chromosomal regions (SSA27 and SSA12) contained single nucleotide polymorphisms associated with resistance to CMS. The highest association signal (P = 6.9751 × 10⁻²⁷) was found on chromosome 27. Four genes with functional roles affecting viral resistance (magi1, pi4kb, bnip2, and ha1f) were found to map closely to the identified quantitative trait loci (QTLs). In conclusion, genetic variation for resistance to CMS was observed in all 3 populations. Two important quantitative trait loci were detected which together explain half of the total genetic variance, suggesting strong potential application for marker-assisted selection and genomic predictions to improve CMS resistance.

The coxsackievirus and adenovirus receptor: virological and biological beauty

The coxsackievirus and adenovirus receptor (CAR) is an essential multifunctional cellular protein that is only beginning to be understood. CAR serves as a receptor for many adenoviruses, human group B coxsackieviruses, swine vesicular disease virus, and possibly other viruses. While named for its function as a viral receptor, CAR is also involved in cell adhesion, immune cell activation, synaptic transmission, and signaling. Knockout mouse models were first to identify some of these biological functions; however, tissue-specific model systems have shed light on the complexity of different CAR isoforms and their specific activities. Many of these functions are mediated by the large number of interacting proteins described so far, and several new putative interactions have recently been discovered. As antiviral and gene therapy strategies that target CAR continue to emerge, future work poised to understand the biological implications of manipulating CAR in vivo is critical.

Isoform specific editing of the coxsackievirus and adenovirus receptor

The Coxsackievirus and adenovirus receptor (CAR) is both a viral receptor and cell adhesion protein. CAR has two transmembrane isoforms that localize distinctly in polarized epithelial cells. Whereas the seven exon-encoded isoform (CAREx7) exhibits basolateral localization, the eight exon-encoded isoform (CAREx8) can localize to the apical epithelial surface where it can mediate luminal adenovirus infection. To further understand the distinct biological functions of these two isoforms, CRISPR/Cas9 genomic editing was used to specifically delete the eighth exon of the CXADR gene in a Madine Darby Canine Kidney (MDCK) cell line with a stably integrated lentiviral doxycycline-inducible CAREx8 cDNA. The gene-edited clone demonstrated a significant reduction in adenovirus susceptibility when both partially and fully polarized, and doxycycline-induction of CAREx8 restored sensitivity to adenovirus. These data reinforce the importance of CAREx8 in apical adenovirus infection and provide a new model cell line to probe isoform specific biological functions of CAR.

Unexpected subcellular distribution of a specific isoform of the Coxsackie and adenovirus receptor, CAR-SIV, in human pancreatic beta cells

AIMS/HYPOTHESIS

The Coxsackie and adenovirus receptor (CAR) is a transmembrane cell-adhesion protein that serves as an entry receptor for enteroviruses and may be essential for their ability to infect cells. Since enteroviral infection of beta cells has been implicated as a factor that could contribute to the development of type 1 diabetes, it is often assumed that CAR is displayed on the surface of human beta cells. However, CAR exists as multiple isoforms and it is not known whether all isoforms subserve similar physiological functions. In the present study, we have determined the profile of CAR isoforms present in human beta cells and monitored the subcellular localisation of the principal isoform within the cells.

METHODS

Formalin-fixed, paraffin-embedded pancreatic sections from non-diabetic individuals and those with type 1 diabetes were studied. Immunohistochemistry, confocal immunofluorescence, electron microscopy and western blotting with isoform-specific antisera were employed to examine the expression and cellular localisation of the five known CAR isoforms. Isoform-specific qRT-PCR and RNA sequencing (RNAseq) were performed on RNA extracted from isolated human islets.

RESULTS

An isoform of CAR with a terminal SIV motif and a unique PDZ-binding domain was expressed at high levels in human beta cells at the protein level. A second isoform, CAR-TVV, was also present. Both forms were readily detected by qRT-PCR and RNAseq analysis in isolated human islets. Immunocytochemical studies indicated that CAR-SIV was the principal isoform in islets and was localised mainly within the cytoplasm of beta cells, rather than at the plasma membrane. Within the cells it displayed a punctate pattern of immunolabelling, consistent with its retention within a specific membrane-bound compartment. Co-immunofluorescence analysis revealed significant co-localisation of CAR-SIV with zinc transporter protein 8 (ZnT8), prohormone convertase 1/3 (PC1/3) and insulin, but not proinsulin. This suggests that CAR-SIV may be resident mainly in the membranes of insulin secretory granules. Immunogold labelling and electron microscopic analysis confirmed that CAR-SIV was localised to dense-core (insulin) secretory granules in human islets, whereas no immunolabelling of the protein was detected on the secretory granules of adjacent exocrine cells. Importantly, CAR-SIV was also found to co-localise with protein interacting with C-kinase 1 (PICK1), a protein recently demonstrated to play a role in insulin granule maturation and trafficking.

CONCLUSIONS/INTERPRETATION

The SIV isoform of CAR is abundant in human beta cells and is localised mainly to insulin secretory granules, implying that it may be involved in granule trafficking and maturation. We propose that this subcellular localisation of CAR-SIV contributes to the unique sensitivity of human beta cells to enteroviral infection.

Poliovirus Receptor: More than a simple viral receptor

The human poliovirus receptor (PVR) is a cell surface protein with a multitude of functions in human biology. PVR was initially identified as the receptor for the human poliovirus and recent discoveries have given a greater insight into both its morphology and its function. Alternative splicing of the PVR gene results in a total of 4 alternatively spliced isoforms. Two of these isoforms lack a complete transmembrane domain and are considered soluble and block viral infection; the remaining two transmembrane isoforms differ only at their extreme C-terminal domains resulting in differential localization in epithelia and polarity of viral infection. In addition to its role as a receptor for the human poliovirus, several native biological functions have also been uncovered. PVR is an important cell adhesion protein and is involved in the transendothelial migration of leukocytes. Through its interactions with CD226 and TIGIT, transmembrane proteins found on leukocytes, PVR is a key regulator of the cell-mediated immune response. As PVR is differentially regulated in a broad spectrum of cancers, it has a strong potential for clinical use as a biomarker. PVR is also a possible target for novel cancer therapies. Utilizing its natural tropism for PVR, a genetically modified form of the live attenuated poliovirus vaccine is currently being tested for its ability to locate and destroy certain tumors. These recent studies emphasize the importance of PVR in human biology and demonstrate its utility beyond being a viral receptor protein.

Adenovirus transduction: More complicated than receptor expression

The abundance and accessibility of a primary virus receptor are critical factors that impact the susceptibility of a host cell to virus infection. The Coxsackievirus and adenovirus receptor (CAR) has two transmembrane isoforms that occur due to alternative splicing and differ in localization and function in polarized epithelia. To determine the relevance of isoform-specific expression across cell types, the abundance and localization of both isoforms were determined in ten common cell lines, and correlated with susceptibility to adenovirus transduction relative to polarized primary human airway epithelia. Data show that the gene and protein expression for each isoform of CAR varies significantly between cell lines and polarization, as indicated by high transepithelial resistance, is inversely related to adenovirus transduction. In summary, the variability of polarity and isoform-specific expression among model cells are critical parameters that must be considered when evaluating the clinical relevance of potential adenovirus-mediated gene therapy and anti-adenovirus strategies.

Cell-cell communication mediated by the CAR subgroup of immunoglobulin cell adhesion molecules in health and disease

The immunoglobulin superfamily represents a diverse set of cell-cell contact proteins and includes well-studied members such as NCAM1, DSCAM, L1 or the contactins which are strongly expressed in the nervous system. In this review we put our focus on the biological function of a less understood subgroup of Ig-like proteins composed of CAR (coxsackievirus and adenovirus receptor), CLMP (CAR-like membrane protein) and BT-IgSF (brain and testis specific immunoglobulin superfamily). The CAR-related proteins are type I transmembrane proteins containing an N-terminal variable (V-type) and a membrane proximal constant (C2-type) Ig domain in their extracellular region which are implicated in homotypic adhesion. They are highly expressed during embryonic development in a variety of tissues including the nervous system whereby in adult stages the protein level of CAR and CLMP decreases, only BT-IgSF expression increases within age. CAR-related proteins are concentrated at specialized cell-cell communication sites such as gap or tight junctions and are present at the plasma membrane in larger protein complexes. Considerable progress has been made on the molecular structure and interactions of CAR while research on CLMP and BT-IgSF is at an early stage. Studies on mouse mutants revealed biological functions of CAR in the heart and for CLMP in the gastrointestinal and urogenital systems. Furthermore, CAR and BT-IgSF appear to regulate synaptic function in the hippocampus.

Tear-mediated delivery of nanoparticles through transcytosis of the lacrimal gland

Rapid clearance from the tears presents a formidable obstacle to the delivery of peptide drugs to the eye surface. This impedes therapies for ocular infections, wound healing, and dry-eye disease that affect the vision of millions worldwide. To overcome this challenge, this manuscript explores a novel strategy to reach the ocular surface via receptor-mediated transcytosis across the lacrimal gland (LG), which produces the bulk of human tears. The LG abundantly expresses the coxsackievirus and adenovirus receptor (CAR); furthermore, we recently reported a peptide-based nanoparticle (KSI) that targets CAR on liver cells. This manuscript reports the unexpected finding that KSI both targets and transcytoses into the LG acinar lumen, which drains to tear ducts. When followed using ex vivo live cell imaging KSI rapidly accumulates in lumen formed by LG acinar cells. LG transduction with a myosin Vb tail, which is dominant negative towards transcytosis, inhibits lumenal accumulation. Transcytosis of KSI was confirmed in vivo by confocal and TEM imaging of LG tissue following administration of KSI nanoparticles. These findings suggest that it is possible to target nanomaterials to the tears by targeting certain receptors on the LG. This design strategy represents a new opportunity to overcome barriers to ocular delivery.

A deletion at ADAMTS9-MAGI1 locus is associated with psoriatic arthritis risk

OBJECTIVE

Copy number variants (CNVs) have been associated with the risk to develop multiple autoimmune diseases. Our objective was to identify CNVs associated with the risk to develop psoriatic arthritis (PsA) using a genome-wide analysis approach.

METHODS

A total of 835 patients with PsA and 1498 healthy controls were genotyped for CNVs using the Illumina HumanHap610 BeadChip genotyping platform. Genomic CNVs were characterised using CNstream analysis software and analysed for association using the χ(2) test. The most significant genomic CNV associations with PsA risk were independently tested in a validation sample of 1133 patients with PsA and 1831 healthy controls. In order to test for the specificity of the variants with PsA aetiology, we also analysed the association to a cohort of 822 patients with purely cutaneous psoriasis (PsC).

RESULTS

A total of 165 common CNVs were identified in the genome-wide analysis. We found a highly significant association of an intergenic deletion between ADAMTS9 and MAGI1 genes on chromosome 3p14.1 (p=0.00014). Using the independent patient and control cohort, we validated the association between ADAMTS9-MAGI1 deletion and PsA risk (p=0.032). Using next-generation sequencing, we characterised the 26 kb associated deletion. Finally, analysing the PsC cohort we found a lower frequency of the deletion compared with the PsA cohort (p=0.0088) and a similar frequency to that of healthy controls (p>0.3).

CONCLUSIONS

The present genome-wide scan for CNVs associated with PsA risk has identified a new deletion associated with disease risk and which is also differential from PsC risk.

Adenovirus entry from the apical surface of polarized epithelia is facilitated by the host innate immune response

Prevention of viral-induced respiratory disease begins with an understanding of the factors that increase or decrease susceptibility to viral infection. The primary receptor for most adenoviruses is the coxsackievirus and adenovirus receptor (CAR), a cell-cell adhesion protein normally localized at the basolateral surface of polarized epithelia and involved in neutrophil transepithelial migration. Recently, an alternate isoform of CAR, CAREx8, has been identified at the apical surface of polarized airway epithelia and is implicated in viral infection from the apical surface. We hypothesized that the endogenous role of CAREx8 may be to facilitate host innate immunity. We show that IL-8, a proinflammatory cytokine and a neutrophil chemoattractant, stimulates the protein expression and apical localization of CAREx8 via activation of AKT/S6K and inhibition of GSK3β. Apical CAREx8 tethers infiltrating neutrophils at the apical surface of a polarized epithelium. Moreover, neutrophils present on the apical-epithelial surface enhance adenovirus entry into the epithelium. These findings suggest that adenovirus evolved to co-opt an innate immune response pathway that stimulates the expression of its primary receptor, apical CAREx8, to allow the initial infection the intact epithelium. In addition, CAREx8 is a new target for the development of novel therapeutics for both respiratory inflammatory disease and adenoviral infection.

1. Alternative splicing of viral receptors: A review of the diverse morphologies and physiologies of adenoviral receptors

Understanding the biology of cell surface proteins is important particularly when they are utilized as viral receptors for viral entry. By manipulating the expression of cell surface receptors that have been coopted by viruses, the susceptibility of an individual to virus-induced disease or, alternatively, the effectiveness of viral-based gene therapy can be modified. The most commonly studied vector for gene therapy is adenovirus. The majority of adenovirus types utilize the coxsackievirus and adenovirus receptor (CAR) as a primary receptor to enter cells. Species B adenovirus do not interact with CAR, but instead interact with the cell surface proteins desmoglein-2 (DSG-2) and cluster of differentiation 46 (CD46). These cell surface proteins exhibit varying degrees of alternative mRNA splicing, creating an estimated 20 distinct protein isoforms. It is likely that alternative splice forms have allowed these proteins to optimize their effectiveness in a plethora of niches, including roles as cell adhesion proteins and regulators of the innate immune system. Interestingly, there are soluble isoforms of these viral receptors, which lack the transmembrane domain. These soluble isoforms can potentially bind to the surface of a virus in the extracellular compartment, blocking the ability of the virus to bind to the host cell, reducing viral infectivity. Finally, the diversity of viral receptor isoforms appears to facilitate an assortment of interactions between viral receptor proteins and cytosolic proteins, leading to differential sorting in polarized cells. Using adenoviral receptors as a model system, the purpose of this review is to highlight the role that isoform-specific protein localization plays in the entry of pathogenic viruses from the apical surface of polarized epithelial cells.

The PDZ3 domain of the cellular scaffolding protein MAGI-1 interacts with the Coxsackievirus and adenovirus receptor (CAR)

The Coxsackievirus and adenovirus receptor (CAR) is an essential cellular protein that is involved in cell-cell adhesion, protein trafficking, and viral infection. The major isoform of CAR is selectively sorted to the basolateral membrane of polarized epithelial cells where it co-localizes with the cellular scaffolding protein membrane-associated guanylate kinase with inverted domain structure-1 (MAGI-1). Previously, we demonstrated CAR interacts with MAGI-1 through a PDZ-domain dependent interaction. Here, we show that the PDZ3 domain of MAGI-1 is exclusively responsible for the high affinity interaction between the seven exon isoform of CAR and MAGI-1 using yeast-two-hybrid analysis and confirming this interaction biochemically and in cellular lysates by in vitro pull down assay and co-immunoprecipitation. The high affinity interaction between the PDZ3 domain and CAR C-terminus was measured by fluorescence resonance energy transfer. Further, we investigated the biological relevance of this high affinity interaction between CAR and the PDZ3 domain of MAGI-1 and found that it does not alter CAR-mediated adenovirus infection. By contrast, interruption of this high affinity interaction altered the localization of MAGI-1 indicating that CAR is able to traffic MAGI-1 to cell junctions. These data deepen the molecular understanding of the interaction between CAR and MAGI-1 and indicate that although CAR plays a role in trafficking PDZ-based scaffolding proteins to cellular junctions, association with a high affinity intracellular binding partner does not significantly alter adenovirus binding and entry via CAR.

Regulation of the blood-testis barrier by coxsackievirus and adenovirus receptor

The blood-testis barrier (BTB) divides the seminiferous epithelium into the basal and the adluminal compartment. It restricts paracellular diffusion of molecules between Sertoli cells, confers cell polarity, and creates a unique microenvironment in the adluminal compartment for spermatid development. However, it undergoes restructuring during the epithelial cycle so that preleptotene spermatocytes differentiated from type B spermatogonia residing in the basal compartment can traverse the BTB at stage VIII of the cycle, while the immunological barrier is maintained. Herein, coxsackievirus and adenovirus receptor (CAR), a tight junction (TJ) integral membrane protein in the testis and multiple epithelia and endothelia, was found to act as a regulatory protein at the BTB, besides serving as a structural adhesion protein. RNAi-mediated knockdown of CAR in a Sertoli cell epithelium with an established TJ-permeability barrier that mimicked the BTB in vivo resulted in a disruption of the TJ barrier and an increase in endocytosis of the TJ-protein occludin. Furthermore, such an enhancement in occludin endocytosis was accompanied by a downregulation of Thr-phosphorylation in occludin and an increase in the association of endocytosed occludin with early endosome antigen-1. These findings were confirmed by overexpressing CAR in Sertoli cells, which was found to "tighten" the Sertoli cell TJ barrier, promoting BTB function. These findings support the emerging concept that CAR is not only a structural protein, it is involved in conferring the phosphorylation status of other adhesion proteins at the BTB (e.g., occludin) possibly mediated via its structural interactions with nonreceptor protein kinases, thereby modulating endocytic vesicle-mediated protein trafficking.