ITPK1 mediates the lipid-independent synthesis of inositol phosphates controlled by metabolism.

Inositol phosphates (IPs) comprise a network of phosphorylated molecules that play multiple signaling roles in eukaryotes. IPs synthesis is believed to originate with IP3 generated from PIP2 by phospholipase C (PLC). Here, we report that in mammalian cells PLC-generated IPs are rapidly recycled to inositol, and uncover the enzymology behind an alternative "soluble" route to synthesis of IPs. Inositol tetrakisphosphate 1-kinase 1 (ITPK1)-found in Asgard archaea, social amoeba, plants, and animals-phosphorylates I(3)P1 originating from glucose-6-phosphate, and I(1)P1 generated from sphingolipids, to enable synthesis of IP6 We also found using PAGE mass assay that metabolic blockage by phosphate starvation surprisingly increased IP6 levels in a ITPK1-dependent manner, establishing a route to IP6 controlled by cellular metabolic status, that is not detectable by traditional [3H]-inositol labeling. The presence of ITPK1 in archaeal clades thought to define eukaryogenesis indicates that IPs had functional roles before the appearance of the eukaryote.

The expanding roles of Gßγ subunits in G protein-coupled receptor signaling and drug action.

Gßγ subunits from heterotrimeric G proteins perform a vast array of functions in cells with respect to signaling, often independently as well as in concert with Gα subunits. However, the eponymous term "Gßγ" does not do justice to the fact that 5 Gß and 12 Gγ isoforms have evolved in mammals to serve much broader roles beyond their canonical roles in cellular signaling. We explore the phylogenetic diversity of Gßγ subunits with a view toward understanding these expanded roles in different cellular organelles. We suggest that the particular content of distinct Gßγ subunits regulates cellular activity, and that the granularity of individual Gß and Gγ action is only beginning to be understood. Given the therapeutic potential of targeting Gßγ action, this larger view serves as a prelude to more specific development of drugs aimed at individual isoforms.

Conformational stability of inositol 1,3,4,5,6-pentakisphosphate 2-kinase (IPK1) dictates its substrate selectivity.

Inositol 1,3,4,5,6-pentakisphosphate 2-kinase (IPK1) converts inositol 1,3,4,5,6-pentakisphosphate(IP5) to inositol hexakisphosphate (IP6). IPK1 shares structural similarity with protein kinases and is suspected to employ a similar mechanism of activation. Previous studies revealed roles for the 1- and 3-phosphates of IP5 in IPK1 activation and revealed that the N-lobe of IPK1 is unstable in the absence of inositol phosphate (IP). Here, we demonstrate the link between IPK1 substrate specificity and the stability of its N-lobe. Limited proteolysis of IPK1 revealed that N-lobe stability is dependent on the presence of the 1-phosphate of the substrate, whereas overall stability of IPK1 was increased in ternary complexes with nucleotide and IPs possessing 1- and 3-phosphates that engage the N-lobe of IPK1. Thus, the 1- and 3-phosphates possess dual roles in both IPK1 activation and IPK1 stability. To test whether kinase stability directly contributed to substrate selectivity of the kinase, we engineered IPK1 mutants with disulfide bonds that artificially stabilized the N-lobe in an IP-independent manner thereby mimicking its substrate-bound state in the absence of IP. IPK1 E82C/S142C exhibited a DTT-sensitive 5-fold increase in kcat for 3,4,5,6-inositol tetrakisphosphate (3,4,5,6-IP4) as compared with wild-type IPK1. The crystal structure of the IPK1 E82C/S142C mutant confirmed the presence of the disulfide bond and revealed a small shift in the N-lobe. Finally, we determined that IPK1 E82C/S142C is substantially more stable than wild-type IPK1 under nonreducing conditions, revealing that increased stability of IPK1 E82C/S142C correlates with changes in substrate specificity by allowing IPs lacking the stabilizing 1-phosphate to be used. Taken together, our results show that IPK1 substrate selection is linked to the ability of each potential substrate to stabilize IPK1.

Roles of phosphate recognition in inositol 1,3,4,5,6-pentakisphosphate 2-kinase (IPK1) substrate binding and activation.

Inositol phosphate kinases (IPKs) sequentially phosphorylate inositol phosphates (IPs) to yield a group of small signaling molecules involved in diverse cellular processes. IPK1 (inositol 1,3,4,5,6-pentakisphosphate 2-kinase) phosphorylates inositol 1,3,4,5,6-pentakisphosphate to inositol 1,2,3,4,5,6-hexakisphosphate; however, the mechanism of IP recognition employed by IPK1 is currently unresolved. We demonstrated previously that IPK1 possesses an unstable N-terminal lobe in the absence of IP, which led us to propose that the phosphate profile of the IP was linked to stabilization of IPK1. Here, we describe a systematic study to determine the roles of the 1-, 3-, 5-, and 6-phosphate groups of inositol 1,3,4,5,6-pentakisphosphate in IP binding and IPK1 activation. The 5- and 6-phosphate groups were the most important for IP binding to IPK1, and the 1- and 3-phosphate groups were more important for IPK1 activation than the others. Moreover, we demonstrate that there are three critical residues (Arg-130, Lys-170, and Lys-411) necessary for IPK1 activity. Arg-130 is the only substrate-binding N-terminal lobe residue that can render IPK1 inactive; its 1-phosphate is critical for full IPK1 activity and for stabilization of the active conformation of IPK1. Taken together, our results support the model for recognition of the IP substrate by IPK1 in which (i) the 4-, 5-, and 6-phosphates are initially recognized by the C-terminal lobe, and subsequently, (ii) the interaction between the 1-phosphate and Arg-130 stabilizes the N-terminal lobe and activates IPK1. This model of IP recognition, believed to be unique among IPKs, could be exploited for selective inhibition of IPK1 in future studies that investigate the role of higher IPs.

Mechanobiology-informed regenerative medicine: Dose-controlled release of placental growth factor from a functionalized collagen-based scaffold promotes angiogenesis and accelerates bone defect healing.

Leveraging the differential response of genes to mechanical loading may allow for the identification of novel therapeutics and we have recently established placental growth factor (PGF) as a mechanically augmented gene which promotes angiogenesis at higher doses and osteogenesis at lower doses. Herein, we sought to execute a mechanobiology-informed approach to regenerative medicine by designing a functionalized scaffold for the dose-controlled delivery of PGF which we hypothesized would be capable of promoting regeneration of critically-sized bone defects. Alginate microparticles and collagen/hydroxyapatite scaffolds were shown to be effective PGF-delivery platforms, as demonstrated by their capacity to promote angiogenesis in vitro. A PGF release profile consisting of an initial burst release to promote angiogenesis followed by a lower sustained release to promote osteogenesis was achieved by incorporating PGF-loaded microparticles into a collagen/hydroxyapatite scaffold already containing directly incorporated PGF. Although this PGF-functionalized scaffold demonstrated only a modest increase in osteogenic capacity in vitro, robust bone regeneration was observed after implantation into rat calvarial defects, indicating that the dose-dependent effect of PGF can be harnessed as an alternative to multi-drug systems for the delivery of both pro-angiogenic and pro-osteogenic cues. This mechanobiology-informed approach provides a framework for strategies aimed at identifying and evaluating novel scaffold-based systems for regenerative applications.

Rapid Implementation of a SARS-CoV-2 Diagnostic Quantitative Real-Time PCR Test with Emergency Use Authorization at a Large Academic Safety Net Hospital.

BACKGROUND: Significant delays in the rapid development and distribution of diagnostic testing for SARS-CoV-2 (COVID-19) infection have prevented adequate public health management of the disease, impacting the timely mapping of viral spread and the conservation of personal protective equipment. Furthermore, vulnerable populations, such as those served by the Boston Medical Center (BMC), the largest safety net hospital in New England, represent a high-risk group across multiple dimensions, including a higher prevalence of pre-existing conditions and substance use disorders, lower health maintenance, unstable housing, and a propensity for rapid community spread, highlighting the urgent need for expedient and reliable in-house testing. METHODS: We developed a SARS-CoV-2 diagnostic medium-throughput qRT-PCR assay with rapid turnaround time and utilized this Clinical Laboratory Improvement Amendments (CLIA)-certified assay for testing nasopharyngeal swab samples from BMC patients, with emergency authorization from the Food and Drug Administration (FDA) and the Massachusetts Department of Public Health. FINDINGS: The in-house testing platform displayed robust accuracy and reliability in validation studies and reduced institutional sample turnaround time from 5-7 days to less than 24 h. Of over 1,000 unique patient samples tested, 44.1% were positive for SARS-CoV-2 infection. CONCLUSIONS: This work provides a blueprint for academic centers and community hospitals lacking automated laboratory machinery to implement rapid in-house testing. FUNDING: This study was supported by funding from the Boston University School of Medicine, the National Institutes of Health, Boston Medical Center, and the Massachusetts Consortium on Pathogen Readiness (MASS CPR).

qRT-PCR Platforms for Diagnosing and Reporting SARS-CoV-2 Infection in Human Samples.

The protocols herein outline the use of qRT-PCR to detect the presence of SARS-CoV-2 genomic RNA in patient samples. In order to cope with potential fluctuations in supply chain and testing demands and to enable expedient adaptation of reagents and assays on hand, we include details for three parallel methodologies (one- and two-step singleplex and one-step multiplex assays). The diagnostic platforms described can be easily adapted by basic science research laboratories for SARS-CoV-2 diagnostic testing with relatively short turnaround time. For complete details on the use and execution of this protocol, please refer to Vanuytsel et al. (2020).

Time-dependent mechanical characterization of poly(2-hydroxyethyl methacrylate) hydrogels using nanoindentation and unconfined compression.

Hydrogels pose unique challenges to nanoindentation including sample preparation, control of experimental parameters, and limitations imposed by mechanical testing instruments and data analysis originally intended for harder materials. The artifacts that occur during nanoindentation of hydrated samples have been described, but the material properties obtained from hydrated nanoindentation have not yet been related to the material properties obtained from macroscale testing. To evaluate the best method for correlating results from microscale and macroscale tests of soft materials, nanoindentation and unconfined compression stress-relaxation tests were performed on poly-2-hydroxyethyl methacrylate (pHEMA) hydrogels with a range of cross-linker concentrations. The nanoindentation data were analyzed with the Oliver-Pharr elastic model and the Maxwell-Wiechert (j = 2) viscoelastic model. The unconfined compression data were analyzed with the Maxwell-Wiechert model. This viscoelastic model provided an excellent fit for the stress-relaxation curves from both tests. The time constants from nanoindentation and unconfined compression were significantly different, and we propose that these differences are due to differences in equilibration time between the microscale and macroscale experiments and in sample geometry. The Maxwell-Wiechert equilibrium modulus provided the best agreement between nanoindentation and unconfined compression. Also, both nanoindentation analyses showed an increase in modulus with each increasing cross-linker concentration, validating that nanoindentation can discriminate between similar, low-modulus, hydrated samples.

Effects of auricular chondrocyte expansion on neocartilage formation in photocrosslinked hyaluronic acid networks.

The overall objective of this study was to examine the effects of in vitro expansion on neocartilage formation by auricular chondrocytes photoencapsulated in a hyaluronic acid (HA) hydrogel as a next step toward the clinical application of tissue engineering therapies for treatment of damaged cartilage. Swine auricular chondrocytes were encapsulated either directly after isolation (p = 0), or after further in vitro expansion ( p = 1 and p = 2) in a 2 wt%, 50-kDa HA hydrogel and implanted subcutaneously in the dorsum of nude mice. After 12 weeks, constructs were explanted for mechanical testing and biochemical and immunohistochemical analysis and compared to controls of HA gels alone and native cartilage. The compressive equilibrium moduli of the p = 0 and p = 1 constructs (51.2 +/- 8.0 and 72.5 +/- 35.2 kPa, respectively) were greater than the p = 2 constructs (26.8 +/- 14.9 kPa) and the control HA gel alone (12.3 +/- 1.3 kPa) and comparable to auricular cartilage (35.1 +/- 12.2 kPa). Biochemical analysis showed a general decrease in glycosaminoglycan (GAG), collagen, and elastin content with chondrocyte passage, though no significant differences were found between the p = 0 and p = 1 constructs for any of the analyses. Histological staining showed intense and uniform staining for aggrecan, as well as greater type II collagen versus type I collagen staining in all constructs. Overall, this study illustrates that constructs with the p = 0 and p = 1 auricular chondrocytes produced neocartilage tissue that resembled native auricular cartilage after 12 weeks in vivo. However, these results indicate that further expansion of the chondrocytes (p = 2) can lead to compromised tissue properties.

PPIP5K1 Suppresses Etoposide-triggered Apoptosis.

Inositol hexakisphosphate kinase 2 (IP6K2) potentiates pro-apoptotic signalling and increases the sensitivity of mammalian cells to cytotoxic agents. Diphosphoinositol pentakisphosphate kinase (PPIP5K) generates inositol pyrophosphates (InsPPs) that are structurally distinct from those produced by IP6K2 and their possible roles in affecting cell viability remain unclear. In the present study, we tested the impact of PPIP5K1 on cellular sensitivity to various genotoxic agents to determine if PPIP5K1 and IP6K2 contribute similarly to apoptosis. We observed that PPIP5K1 overexpression decreased sensitivity of cells toward several cytotoxic agents, including etoposide, cisplatin, and sulindac. We further tested the impact of PPIP5K1 overexpression on an array of apoptosis markers and observed that PPIP5K1 decreased p53 phosphorylation on key residues, including Ser-15, -46, and -392. Overexpression of a kinase-impaired PPIP5K1 mutant failed to protect cells from apoptosis, indicating this protection is a consequence PPIP5K1 catalytic activity, in contrast with the sensitivity conferred by IP6K2, which is dependent on both catalytic and non-catalytic functions. These observations reveal distinct roles for PPIP5K1 and IP6K2 and the InsPPs they produce in controlling cell death.

Mass spectrometry analysis of PPIP5K1 interactions and data on cell motility of PPIP5K1-deficient cells.

Inositol pyrophosphates are cellular signals that are created by the actions of inositol kinases and are degraded by highly active inositol phosphatases. The potent actions of these phosphatases suggest these signals must be created near their sites of action. To identify sites where the inositol kinase, PPIP5K1 acts, we performed affinity purification of PPIP5K1 from HEK293 cells and analyzed these samples using mass spectrometry to identify the proteins pesent (10.1016/j.cellsig.2016.02.002) [1]. We further decreased PPIP5K1 levels in HeLa cells and treated these with PPIP5K1 siRNA. We then monitored the motility of these cells in Scratch assays.

PPIP5K1 interacts with the exocyst complex through a C-terminal intrinsically disordered domain and regulates cell motility.

Cellular signaling involves coordinated regulation of many events. Scaffolding proteins are crucial regulators of cellular signaling, because they are able to affect numerous events by coordinating specific interactions among multiple protein partners in the same pathway. Scaffolding proteins often contain intrinsically disordered regions (IDR) that facilitate the formation and function of distinct protein complexes. We show that PPIP5K1 contains an unusually long and evolutionarily conserved IDR. To investigate the biological role(s) of this domain, we identified interacting proteins using affinity purification coupled with mass spectrometry. Here, we report that PPIP5K1 is associated with a network of proteins that regulate vesicle-mediated transport. We further identified exocyst complex component 1 as a direct interactor with the IDR of PPIP5K1. Additionally, we report that knockdown of PPIP5K1 decreases motility of HeLa cells in a wound-healing assay. These results suggest that PPIP5K1 might play an important role in regulating function of exocyst complex in establishing cellular polarity and directional migration of cells.

Mechanical microenvironments and protein expression associated with formation of different skeletal tissues during bone healing.

Uncovering the mechanisms of the sensitivity of bone healing to mechanical factors is critical for understanding the basic biology and mechanobiology of the skeleton, as well as for enhancing clinical treatment of bone injuries. This study refined an experimental method of measuring the strain microenvironment at the site of a bone injury during bone healing. This method used a rat model in which a well-controlled bending motion was applied to an osteotomy to induce the formation of pseudarthrosis that is composed of a range of skeletal tissues, including woven bone, cartilage, fibrocartilage, fibrous tissue, and clot tissue. The goal of this study was to identify both the features of the strain microenvironment associated with formation of these different tissues and the expression of proteins frequently implicated in sensing and transducing mechanical cues. By pairing the strain measurements with histological analyses that identified the regions in which each tissue type formed, we found that formation of the different tissue types occurs in distinct strain microenvironments and that the type of tissue formed is correlated most strongly to the local magnitudes of extensional and shear strains. Weaker correlations were found for dilatation. Immunohistochemical analyses of focal adhesion kinase and rho family proteins RhoA and CDC42 revealed differences within the cartilaginous tissues in the calluses from the pseudarthrosis model as compared to fracture calluses undergoing normal endochondral bone repair. These findings suggest the involvement of these proteins in the way by which mechanical stimuli modulate the process of cartilage formation during bone healing.

Inositol phosphate-induced stabilization of inositol 1,3,4,5,6-pentakisphosphate 2-kinase and its role in substrate specificity.

Inositol phosphate kinases (IPKs) sequentially phosphorylate inositol phosphates (IPs) on their inositol rings to yield an array of signaling molecules. IPKs must possess the ability to recognize their physiological substrates from among a pool of over 30 cellular IPs that differ in numbers and positions of phosphates. Crystal structures from IPK subfamilies have revealed structural determinants for IP discrimination, which vary considerably between IPKs. However, recent structures of inositol 1,3,4,5,6-pentakisphosphate 2-kinase (IPK1) did not reveal how IPK1 selectively recognizes its physiological substrate, IP5, while excluding others. Here, we report that limited proteolysis has revealed the presence of multiple conformational states in the IPK1 catalytic cycle, with notable protection from protease only in the presence of IP. Further, a 3.1-Å crystal structure of IPK1 bound to ADP in the absence of IP revealed decreased order in residues 110-140 within the N-lobe of the kinase compared with structures in which IP is bound. Using this solution and crystallographic data, we propose a model for recognition of IP substrate by IPK1 wherein phosphate groups at the 4-, 5-, and 6-positions are recognized initially by the C-lobe with subsequent interaction of the 1-position phosphate by Arg130 that stabilizes this residue and the N-lobe. This model explains how IPK1 can be highly specific for a single IP substrate by linking its interactions with substrate phosphate groups to the stabilization of the N- and C-lobes and kinase activation.

Activin A regulates porcine follicle-stimulating hormone beta-subunit transcription via cooperative actions of SMADs and FOXL2.

Activins stimulate FSH synthesis and secretion by pituitary gonadotrope cells. Activin A induction of porcine and murine FSHß (Fshb) gene transcription in immortalized gonadotropes is dependent on homolog of Drosophila mothers against decapentaplegic (SMAD) proteins as well as the forkhead transcription factor L2 (FOXL2). Using both heterologous and homologous cell models, we demonstrate that FOXL2 functionally synergizes with SMAD3/4 to stimulate porcine Fshb promoter-reporter activity. We further show that endogenous FOXL2 and SMAD2/3 physically interact in homologous cells. We identify two composite cis-elements of adjacent FOXL2 and SMAD binding sites in the proximal porcine Fshb promoter that mediate activin A, FOXL2, and SMAD3 actions. FOXL2 can bind these elements independently of SMADs, whereas SMAD3/4 binding requires high-affinity FOXL2 binding. Conversely, FOXL2 alone is insufficient to regulate Fshb transcription and requires SMADs to induce promoter activity. Collectively, our data suggest a model in which activins stimulate formation and nuclear accumulation of SMAD3/4 complexes, which interact with FOXL2 bound to at least two proximal promoter elements. This association stabilizes SMAD3/4 binding to adjacent SMAD binding elements. SMAD-FOXL2 complexes then mediate activation of transcription through a currently unknown mechanism. Conservation of one of the two composite cis-elements suggests that this may form part of a general mechanism whereby activins regulate Fshb subunit transcription and FSH synthesis.

Serum-dependent effects on adult and fetal tendon fibroblast migration and collagen expression.

Cell migration and extracellular matrix synthesis play an important role in the wound-healing response to injury. Several studies have described differences in migratory behavior and collagen biosynthetic activity in adult vs. fetal skin fibroblasts. The objective of this study was to examine the serum- and age-dependent effects on cell migration and collagen expression in tendon fibroblasts. Medial tendon fibroblasts were isolated from pregnant ewes and their fetuses, and cultured with and without serum for up to 7 days. Cell migration was determined by quantitative image analysis, and collagen expression was assessed by reverse transcription-polymerase chain reaction and immunohistochemical staining. In serum-containing medium, tendon fibroblasts migrated significantly faster than cells in serum-free medium. Additionally, fetal tendon fibroblasts migrated significantly faster than adult tendon fibroblasts under both culture conditions. The expression of types I and III collagen mRNA was significantly up-regulated in tendon cell populations in serum-free medium compared with those in serum-containing medium. Quantitative assessment of collagen staining indicated that fetal tenocytes produced more type I collagen than adult tenocytes under both culture conditions. These findings suggest that there is an inherent difference between adult and fetal tendon fibroblasts, which may have implications in the wound-healing response in tendons.

The effect of serial monolayer passaging on the collagen expression profile of outer and inner anulus fibrosus cells.

STUDY DESIGN: Sheep outer and inner anulus fibrosus cells were isolated and analyzed to determine the effect of serial monolayer passaging on their phenotype. OBJECTIVES: To characterize the effect of sequential serial passage on outer and inner anulus cells to determine at which point passaged cells are significantly different from freshly isolated cells. SUMMARY OF BACKGROUND DATA: Previous studies show that chondrocytic cells lose their differentiated phenotype with sequential monolayer passage. Although intervertebral disc cells are similar, to our knowledge, a complete characterization of passage effects has not been performed. METHODS: Sheep outer and inner anulus cells were isolated, serially passaged, and evaluated for changes in cellular morphology, collagen I and II gene expression and protein elaboration, and total protein and deoxyribonucleic acid content. RESULTS: Outer anulus cells displayed an elongated morphology, while inner anulus cells were initially polygonal and became more fibroblast-like with passage. At low passage, outer anulus cells showed higher collagen I expression, while inner anulus cells indicated higher collagen II expression. At high passage, collagen I expression increased for inner anulus cells and decreased for outer anulus cells, whereas collagen II expression decreased for both cell types. Immunohistochemical staining confirmed gene expression results. CONCLUSIONS: The differences in expression profiles of outer and inner anulus cells support previous findings that zonal differences exist between the cell types. Up to passage 2, both cell types were not significantly different from freshly isolated cells and maintained distinct phenotypic characteristics. However, after 6 sequential passages, outer and inner anulus cells became morphologically indistinguishable, and displayed no significant differences in collagen I gene and protein expression, thus becoming a more homogeneous population. As such, serial monolayer passaging has a marked effect on disc cell behavior, and is an important factor to consider when designing and evaluating in vitro studies and for potential cell-based therapies for disc repair.

Specificity determinants in inositol polyphosphate synthesis: crystal structure of inositol 1,3,4-trisphosphate 5/6-kinase.

Inositol hexakisphosphate and other inositol high polyphosphates have diverse and critical roles in eukaryotic regulatory pathways. Inositol 1,3,4-trisphosphate 5/6-kinase catalyzes the rate-limiting step in inositol high polyphosphate synthesis in animals. This multifunctional enzyme also has inositol 3,4,5,6-tetrakisphosphate 1-kinase and other activities. The structure of an archetypal family member, from Entamoeba histolytica, has been determined to 1.2 A resolution in binary and ternary complexes with nucleotide, substrate, and product. The structure reveals an ATP-grasp fold. The inositol ring faces ATP edge-on such that the 5- and 6-hydroxyl groups are nearly equidistant from the ATP gamma-phosphate in catalytically productive phosphoacceptor positions and explains the unusual dual site specificity of this kinase. Inositol tris- and tetrakisphosphates interact via three phosphate binding subsites and one solvent-exposed site that could in principle be occupied by 18 different substrates, explaining the mechanisms for the multiple specificities and catalytic activities of this enzyme.

Crystal structure of the catalytic core of inositol 1,4,5-trisphosphate 3-kinase.

Soluble inositol polyphosphates are ubiquitous second messengers in eukaryotes, and their levels are regulated by an array of specialized kinases. The structure of an archetypal member of this class, inositol 1,4,5-trisphosphate 3-kinase (IP3K), has been determined at 2.2 angstroms resolution in complex with magnesium and adenosine diphosphate. IP3K contains a catalytic domain that is a variant of the protein kinase superfamily, and a novel four-helix substrate binding domain. The two domains are in an open conformation with respect to each other, suggesting that substrate recognition and catalysis by IP3K involves a dynamic conformational cycle. The unique helical domain of IP3K blocks access to the active site by membrane-bound phosphoinositides, explaining the structural basis for soluble inositol polyphosphate specificity.

Recognition of accessory protein motifs by the gamma-adaptin ear domain of GGA3.

Adaptor proteins load transmembrane protein cargo into transport vesicles and serve as nexuses for the formation of large multiprotein complexes on the nascent vesicles. The gamma-adaptin ear (GAE) domains of the AP-1 adaptor protein complex and the GGA adaptor proteins recruit accessory proteins to these multiprotein complexes by binding to a hydrophobic motif. We determined the structure of the GAE domain of human GGA3 in complex with a peptide based on the DFGPLV sequence of the accessory protein Rabaptin-5 and refined it at a resolution of 2.2 A. The leucine and valine residues of the peptide are partly buried in two contiguous shallow, hydrophobic depressions. The anchoring phenylalanine is buried in a deep pocket formed by the aliphatic portions of two conserved arginine residues, along with an alanine and a proline, illustrating the unusual function of a cluster of basic residues in binding a hydrophobic motif.