The bovine papillomavirus E5 protein and the PDGF β receptor: It takes two to tango

Self Cite

This work describes a genetic approach to isolate small, artificial transmembrane (TM) proteins with biological activity. The bovine papillomavirus E5 protein is a dimeric, 44-amino acid TM protein that transforms cells by specifically binding and activating the platelet-derived growth factor β receptor (PDGFβR). We used the E5 protein as a scaffold to construct a retrovirus library expressing ∼500; 000 unique 44-amino acid proteins with randomized TM domains. We screened this library to select small, dimeric TM proteins that were structurally unrelated to erythropoietin (EPO), but specifically activated the human EPO receptor (hEPOR). These proteins did not activate the murine EPOR or the PDGFβR. Genetic studies with one of these activators suggested that it interacted with the TM domain of the hEPOR. Furthermore, this TM activator supported erythroid differentiation of primary human hematopoietic progenitor cells in vitro in the absence of EPO. Thus, we have changed the specificity of a protein so that it no longer recognizes its natural target but, instead, modulates an entirely different protein. This represents a novel strategy to isolate small artificial proteins that affect diverse membrane proteins. We suggest the word "traptamer" for these transmembrane aptamers.bovine papilloma virus | E5 protein | expression libraries | protein engineering | traptamers I t is thought that up to 30% of all proteins span cell membranes (1). Most membrane-spanning protein segments adopt an α-helical conformation that is largely independent of their amino acid sequence and minimally influenced by extramembraneous protein domains. Although transmembrane (TM) domains were once thought to be rather nonspecific protein segments whose primary function was to anchor proteins in membranes, they can participate in highly-specific, inter-and intramolecular side-by-side interactions that control important biological processes (2).The 44-amino acid bovine papillomavirus type 1 (BPV) E5 protein is essentially an isolated TM domain. It contains a hydrophobic central segment and exists in the intracellular membranes of transformed cells as a type II TM homodimer stabilized by disulfide bonds in its carboxy-terminus (3-5). The E5 dimer binds to the TM domains of two molecules of the platelet-derived growthfactor β receptor (PDGFβR) tyrosine kinase, resulting in receptor dimerization and activation, leading to cell transformation (6-10). The E5 protein does not bind to other TM domains or to certain PDGFβR TM mutants (11).Based on our analysis of the E5 protein, we proposed that it might be possible to construct small, artificial TM proteins that regulate a variety of viral and cellular TM proteins in trans (11, 12). To identify small proteins with different TM sequences that transform cells, we constructed expression libraries in which the TM domain of the E5 protein was replaced with randomized sequences of primarily hydrophobic amino acids and isolated clones from these libraries that induced focus formation or growth-factor independe...

mentioning

confidence: 99%

mentioning

confidence: 99%

Construction and genetic selection of small transmembrane proteins that activate the human erythropoietin receptor

Cammett¹,

Jun²,

Cohen³

et al. 2010

Self Cite

“…We speculate that the isoleucine side-chain at position 13 makes an essential packing contact with the hydrophilic threonine residue at position 513 in the middle of the PDGFβR transmembrane domain, a residue that is required for traptamer activity (Fig. 2D) and is proposed to interact directly with Gln17 of the E5 protein (21). Alternatively, these traptamers may act as homodimers, and Ile13 may allow the selfassociation of two molecules of the traptamer to generate an active dimer.…”

Section: Discussionmentioning

confidence: 99%

“…The dimeric 44-amino acid bovine papillomavirus (BPV) E5 protein is the smallest known naturally occurring oncoprotein (14)(15)(16). This very hydrophobic protein, essentially a free-standing transmembrane domain with short extramembraneous segments, binds specifically to the transmembrane domain of the PDGFβR, resulting in constitutive activation of this receptor (14,(16)(17)(18)(19)(20)(21). Sustained mitogenic signaling by the activated PDGFβR confers the transformed phenotype on cells, including loss of contact inhibition, morphologic transformation, focus formation, growth factor independence, and tumorigenicity in animals.…”

mentioning

confidence: 99%

Biologically active LIL proteins built with minimal chemical diversity

Heim

Marston

Federman

et al. 2015

Self Cite

We have constructed 26-amino acid transmembrane proteins that specifically transform cells but consist of only two different amino acids. Most proteins are long polymers of amino acids with 20 or more chemically distinct side-chains. The artificial transmembrane proteins reported here are the simplest known proteins with specific biological activity, consisting solely of an initiating methionine followed by specific sequences of leucines and isoleucines, two hydrophobic amino acids that differ only by the position of a methyl group. We designate these proteins containing leucine (L) and isoleucine (I) as LIL proteins. These proteins functionally interact with the transmembrane domain of the platelet-derived growth factor β-receptor and specifically activate the receptor to transform cells. Complete mutagenesis of these proteins identified individual amino acids required for activity, and a protein consisting solely of leucines, except for a single isoleucine at a particular position, transformed cells. These surprisingly simple proteins define the minimal chemical diversity sufficient to construct proteins with specific biological activity and change our view of what can constitute an active protein in a cellular context.T he chemical diversity of the 20 standard amino acid sidechains found in proteins supports myriad biochemical activities essential for life, and additional chemical diversity is generated by posttranslational amino acid modifications. The number of potential protein sequences is enormous: for proteins only 300 amino acids long, ∼10 400 possible sequences exist, a number larger by many orders-of-magnitude than the number of atoms in the known universe. This immense number of sequences and the chemical and conformational complexity of naturally occurring proteins hinder our ability to understand protein structure, folding, and function, and complicate protein engineering efforts. In addition, many different related amino acid sequences can fold into nearly identical structures, and even proteins with quite divergent sequences or protein folds can use similar chemistry to carry out related functions or display the same catalytic activity (e.g., refs. 1-3). These considerations further complicate attempts to clearly identify and understand the key structural features of proteins. Thus, the isolation of biologically active proteins with drastically reduced chemical complexity would represent a significant advance in protein science by facilitating the correlation of specific structural features with function.Up to 30% of all proteins contain ∼20-25-amino acid, primarily hydrophobic segments that span cell membranes (4). These transmembrane domains often play critical roles in cellular processes by engaging in highly specific protein-protein and protein-lipid interactions required for the proper folding, oligomerization, and function of transmembrane proteins (5-8). For example, receptor tyrosine kinases (RTKs) such as the PDGF-β receptor (PDGFβR) usually consist of an extracellular ligand binding do...

“…The E5 protein transforms cells by activating the cellular platelet-derived growth factor (PDGF) β receptor (PDGFβR), although there may be additional minor, alternative transforming pathways as well (13). The PDGFβR is a receptor tyrosine kinase (RTK) with an extracellular domain that binds PDGF, a hydrophobic membrane-spanning segment, and a cytoplasmic domain with tyrosine kinase activity.…”

mentioning

confidence: 99%

Two transmembrane dimers of the bovine papillomavirus E5 oncoprotein clamp the PDGF β receptor in an active dimeric conformation

Karabadzhak

Petti

Barrera

et al. 2017

Self Cite

The dimeric 44-residue E5 protein of bovine papillomavirus is the smallest known naturally occurring oncoprotein. This transmembrane protein binds to the transmembrane domain (TMD) of the platelet-derived growth factor β receptor (PDGFβR), causing dimerization and activation of the receptor. Here, we use Rosetta membrane modeling and all-atom molecular dynamics simulations in a membrane environment to develop a chemically detailed model of the E5 protein/PDGFβR complex. In this model, an active dimer of the PDGFβR TMD is sandwiched between two dimers of the E5 protein. Biochemical experiments showed that the major PDGFβR TMD complex in mouse cells contains two E5 dimers and that binding the PDGFβR TMD to the E5 protein is necessary and sufficient to recruit both E5 dimers into the complex. These results demonstrate how E5 binding induces receptor dimerization and define a molecular mechanism of receptor activation based on specific interactions between TMDs.transmembrane protein complex | oncogene | traptamer | BPV | blue native gel electrophoresis B ecause viruses modulate signaling nodes that control cell behavior and virus replication, the study of viral proteins has provided great insight into many aspects of cellular biochemistry and the cellular processes that regulate biological function. Thus, viral proteins have long been recognized as valuable tools to probe central problems in biology. A particularly interesting viral protein is the 44-residue E5 oncoprotein encoded by bovine papillomavirus type 1 (BPV). The BPV E5 protein and closely related E5 proteins of other fibropapillomaviruses are the shortest known, naturally occurring proteins with tumorigenic potential (1). The E5 protein is an extremely hydrophobic integral membrane protein located primarily in the membranes of the Golgi apparatus of transformed cells (2, 3). Biophysical studies in model membranes indicate that the E5 protein adopts a transmembrane orientation roughly perpendicular to the membrane surface (4-6). In essence, the E5 protein is a freestanding transmembrane domain (TMD), with a type II transmembrane orientation in which a short C-terminal segment protrudes into the lumen of the Golgi (2). In cells, detergent micelles, and model lipid bilayers, the E5 protein exists as a homodimer stabilized by disulfide bonds involving C-terminal cysteine residues (3-5, 7-10). Genetic studies showed that E5 dimerization is required for transforming activity, and a preferred orientation of the E5 dimer with a symmetric homodimer interface has been identified (7,9,11,12).The E5 protein transforms cells by activating the cellular platelet-derived growth factor (PDGF) β receptor (PDGFβR), although there may be additional minor, alternative transforming pathways as well (13). The PDGFβR is a receptor tyrosine kinase (RTK) with an extracellular domain that binds PDGF, a hydrophobic membrane-spanning segment, and a cytoplasmic domain with tyrosine kinase activity. The inactive PDGFβR is primarily monomeric in unstimulated cells, and PDGF binding indu...