Anthony Bretscher

ERM proteins and merlin: integrators at the cell cortex.

A fundamental property of many plasma-membrane proteins is their association with the underlying cytoskeleton to determine cell shape, and to participate in adhesion, motility and other plasma-membrane processes, including endocytosis and exocytosis. The ezrin–radixin–moesin (ERM) proteins are crucial components that provide a regulated linkage between membrane proteins and the cortical cytoskeleton, and also participate in signal-transduction pathways. The closely related tumour suppressor merlin shares many properties with ERM proteins, yet also provides a distinct and essential function.

A kinase-regulated PDZ-domain interaction controls endocytic sorting of the beta2-adrenergic receptor.

A fundamental question in cell biology is how membrane proteins are sorted in the endocytic pathway. The sorting of internalized β2-adrenergic receptors between recycling endosomes and lysosomes is responsible for opposite effects on signal transduction and is regulated by physiological stimuli. Here we describe a mechanism that controls this sorting operation, which is mediated by a family of conserved protein-interaction modules called PDZ domains. The phosphoprotein EBP50 (for ezrin–radixin–moesin(ERM)-binding phosphoprotein-50) binds to the cytoplasmic tail of the β2-adrenergic receptor through a PDZ domain and to the cortical actin cytoskeleton through an ERM-binding domain. Disrupting the interaction of EBP50 with either domain or depolymerization of the actin cytoskeleton itself causes missorting of endocytosed β2-adrenergic receptors but does not affect the recycling of transferrin receptors. A serine residue at position 411 in the tail of the β2-adrenergic receptor is a substrate for phosphorylation by GRK-5 (for G-protein-coupled-receptor kinase-5) (ref. 5) and is required for interaction with EBP50 and for proper recycling of the receptor. Our results identify a new role for PDZ-domain-mediated protein interactions and for the actin cytoskeleton in endocytic sorting, and suggest a mechanism by which GRK-mediated phosphorylation could regulate membrane trafficking of G-protein-coupled receptors after endocytosis.

Organizing the cell cortex: the role of ERM proteins

Specialized membrane domains are an important feature of almost all cells. In particular, they are essential to tissues that have a highly organized cell cortex, such as the intestinal brush border epithelium. The ERM proteins (ezrin, radixin and moesin) have a crucial role in organizing membrane domains through their ability to interact with transmembrane proteins and the cytoskeleton. In doing so, they can provide structural links to strengthen the cell cortex and regulate the activities of signal transduction pathways. Recent studies examining the structure and in vivo functions of ERMs have greatly advanced our understanding of the importance of membrane–cytoskeleton interactions.

Structure of the ERM Protein Moesin Reveals the FERM Domain Fold Masked by an Extended Actin Binding Tail Domain

The ezrin-radixin-moesin (ERM) protein family link actin filaments of cell surface structures to the plasma membrane, using a C-terminal F-actin binding segment and an N-terminal FERM domain, a common membrane binding module. ERM proteins are regulated by an intramolecular association of the FERM and C-terminal tail domains that masks their binding sites. The crystal structure of a dormant moesin FERM/tail complex reveals that the FERM domain has three compact lobes including an integrated PTB/PH/ EVH1 fold, with the C-terminal segment bound as an extended peptide masking a large surface of the FERM domain. This extended binding mode suggests a novel mechanism for how different signals could produce varying levels of activation. Sequence conservation suggests a similar regulation of the tumor suppressor merlin.

Regulation of cortical structure by the ezrin-radixin-moesin protein family

Molecules involved in ERM (ezrin-radixin-moesin) based attachment of membrane proteins to the cortical cytoskeleton in cell surface structures have been identified. In lymphocytes, a direct interaction is seen with extracellular matrix receptors and intercellular adhesion molecules. In polarized epithelial cells, an adaptor molecule named EBP50 provides a bridge between the amino-terminal domain of ezrin and the cytoplasmic regions of plasma membrane proteins, including the cystic fibrosis transmembrane conductance regulator (CFTR) and the beta2 adrenergic receptor. ERM proteins are conformationally regulated - binding sites for EBP50 and F actin are masked in the dormant molecules and activation leads to exposure of these sites. The mechanism of activation, however, remains to be fully elucidated. ERM proteins also play a role in the Rho and Rac signaling pathways: activated ERM proteins can dissociate Rho-GDI (GDP dissociation inhibitor) from Rho and thereby activate Rho-dependent pathways.

cDNA cloning and sequencing of the protein-tyrosine kinase substrate, ezrin, reveals homology to band 4.1.

Ezrin is a component of the microvilli of intestinal epithelial cells and serves as a major cytoplasmic substrate for certain protein‐tyrosine kinases. We have cloned and sequenced a human ezrin cDNA and report here the entire protein sequence derived from the nucleotide sequence of the cDNA as well as from partial direct protein sequencing. The deduced protein sequence indicates that ezrin is a highly charged protein with an overall pI of 6.1 and a calculated molecular mass of 69,000. The cDNA clone was used to survey the distribution of the ezrin transcript, and the 3.2 kb ezrin mRNA was found to be expressed in the same tissues that are known to express the protein and at the same relative levels. Highest expression was found in intestine, kidney and lung. The cDNA clone hybridized to DNAs from widely divergent organisms indicating that its sequence is highly conserved throughout evolution. The amino acid sequence of ezrin revealed a high degree of similarity within its N‐terminal domain to the erythrocyte cytoskeletal protein, band 4.1 and secondary structure predictions indicate that a second region of ezrin contains a long alpha‐helix, a feature also common to band 4.1. The structural similarity of ezrin to band 4.1 suggests a mechanism for the observed localization to the membrane, and a role for ezrin in modulating the association of the cortical cytoskeleton with the plasma membrane.

Disruption of the single tropomyosin gene in yeast results in the disappearance of actin cables from the cytoskeleton

The yeast tropomyosin gene, designated TPM1, is present in a single copy per haploid genome and encodes a protein with a predicted molecular weight of 23.5 kd. The protein sequence is homologous to higher cell tropomyosins, including the characteristic hydrophobic-hydrophilic pseudoheptapeptide repeats. Indirect immunofluorescence microscopy reveals that tropomyosin is localized with actin cables in wild-type cells. Disruption of TPM1 is not lethal, but results in a reduced growth rate and disappearance of actin cables. Strains carrying the conditional actin mutation act1-2 also lack actin cables; overexpression of tropomyosin in these strains partially restores actin cables. These results strongly suggest that tropomyosin interacts with F actin in vivo and may play an important role in assembling or stabilizing actin cables in yeast.

Secretory vesicle transport velocity in living cells depends on the myosin-V lever arm length

Myosins are molecular motors that exert force against actin filaments. One widely conserved myosin class, the myosin-Vs, recruits organelles to polarized sites in animal and fungal cells. However, it has been unclear whether myosin-Vs actively transport organelles, and whether the recently challenged lever arm model developed for muscle myosin applies to myosin-Vs. Here we demonstrate in living, intact yeast that secretory vesicles move rapidly toward their site of exocytosis. The maximal speed varies linearly over a wide range of lever arm lengths genetically engineered into the myosin-V heavy chain encoded by the MYO2 gene. Thus, secretory vesicle polarization is achieved through active transport by a myosin-V, and the motor mechanism is consistent with the lever arm model.

The Carboxyl-terminal Region of EBP50 Binds to a Site in the Amino-terminal Domain of Ezrin That Is Masked in the Dormant Molecule

EBP50 (ezrin-radixin-moesin-bindingphosphoprotein 50) was recently identified by affinity chromatography on the immobilized NH2-terminal domain of ezrin. Here we map and characterize the regions in EBP50 and ezrin necessary for this association. Using blot overlays and in solution binding assays, the COOH-terminal 30 residues of EBP50 were found to be sufficient for an association with residues 1–286 of ezrin. EBP50 did not bind to full-length (1–585) ezrin, indicating that the EBP50 binding site is masked in the full-length molecule. Ezrin contains two complementary self-association domains known as N- and C-ERMADs (ezrin-radixin-moesin-associationdomains), encompassing residues 1–296 and 479–585, respectively. An ezrin 1–583 construct lacking the two terminal residues necessary for this association was found to have an unmasked EBP50 binding site. Moreover, binding of EBP50 and the C-ERMAD to ezrin residues 1–296 was found to be mutually exclusive, with the C-ERMAD having a higher affinity. These results suggest that in full-length ezrin, the binding site for EBP50 is masked through an intramolecular N/C-ERMAD association. Based on these and additional results, we propose a model whereby dormant ezrin can be activated to bind EBP50 on its NH2-terminal end and F-actin on its COOH-terminal end. Since EBP50 is proposed to bind membrane proteins through its PDZ domains, this provides a molecular description of the regulated linkage of microfilaments to membranes in cell surface microvilli.

Polarized growth and organelle segregation in yeast: the tracks, motors, and receptors

In yeast, growth and organelle segregation requires formin-dependent assembly of polarized actin cables. These tracks are used by myosin Vs to deliver secretory vesicles for cell growth, organelles for their segregation, and mRNA for fate determination. Several specific receptors have been identified that interact with the cargo-binding tails of the myosin Vs. A recent study implicates specific degradation in the bud of the vacuolar receptor, Vac17, as a mechanism for cell cycle–regulated segregation of this organelle.