Extracellular protein:protein interactions between secreted or membrane-tethered proteins are critical for both initiating intercellular communication and ensuring cohesion within multicellular organisms. Proteins predicted to form extracellular interactions are encoded by approximately a quarter of human genes, but despite their importance and abundance, the majority of these proteins have no documented binding partner. Primarily, this is due to their biochemical intractability: membrane-embedded proteins are difficult to solubilise in their native conformation and contain structurally-important posttranslational modifications. Also, the interaction affinities between receptor proteins are often characterised by extremely low interaction strengths (half-lives < 1 second) precluding their detection with many commonly-used high throughput methods. Here, we describe an assay, AVEXIS (AVidity-based EXtracellular Interaction Screen) that overcomes these technical challenges enabling the detection of very weak protein interactions (t(1/2) ≤ 0.1 sec) with a low false positive rate. The assay is usually implemented in a high throughput format to enable the systematic screening of many thousands of interactions in a convenient microtitre plate format (Fig. 1). It relies on the production of soluble recombinant protein libraries that contain the ectodomain fragments of cell surface receptors or secreted proteins within which to screen for interactions; therefore, this approach is suitable for type I, type II, GPI-linked cell surface receptors and secreted proteins but not for multipass membrane proteins such as ion channels or transporters. The recombinant protein libraries are produced using a convenient and high-level mammalian expression system, to ensure that important posttranslational modifications such as glycosylation and disulphide bonds are added. Expressed recombinant proteins are secreted into the medium and produced in two forms: a biotinylated bait which can be captured on a streptavidin-coated solid phase suitable for screening, and a pentamerised enzyme-tagged (β-lactamase) prey. The bait and prey proteins are presented to each other in a binary fashion to detect direct interactions between them, similar to a conventional ELISA (Fig. 1). The pentamerisation of the proteins in the prey is achieved through a peptide sequence from the cartilage oligomeric matrix protein (COMP) and increases the local concentration of the ectodomains thereby providing significant avidity gains to enable even very transient interactions to be detected. By normalising the activities of both the bait and prey to predetermined levels prior to screening, we have shown that interactions having monomeric half-lives of 0.1 sec can be detected with low false positive rates.

The chemokine receptor, CCR5, acts as a co-receptor for human immunodeficiency virus entry into cells. CCR5 has been shown to be targeted to cholesterol- and sphingolipid-rich membrane microdomains termed lipid rafts or caveolae. Cholesterol is essential for CCL4 binding to CCR5 and for keeping the conformational integrity of the receptor. Filipin treatment leads to loss of caveolin-1 from the membrane and therefore to a collapse of the caveolae. We have found here that sequestration of membrane cholesterol with filipin did not affect receptor signalling, however a loss of ligand-induced internalisation of CCR5 was observed. Cholesterol extraction with methyl-beta-cyclodextrin (MCD) reduced signalling through CCR5 as measured by release of intracellular Ca(2+) and completely abolished the inhibition of forskolin-stimulated cAMP accumulation with no effect on internalisation. Pertussis toxin (PTX) treatment inhibited the intracellular release of calcium that is transduced via Galphai G-proteins. Depletion of cholesterol destroyed microdomains in the membrane and switched CCR5/G-protein coupling to a PTX-independent G-protein. We conclude that cholesterol in the membrane is essential for CCR5 signalling via the Galphai G-protein subunit, and that integrity of lipid rafts is not essential for effective CCR5 internalisation however it is crucial for proper CCR5 signal transduction via Galphai G-proteins.

CC and CXC chemokine receptor signalling networks are regulated in different ways. Here we show that intracellular calcium release and cell migration occur independent of Gβγ activation in response to CCL3, whereas CXCL11 induced migration of activated T-lymphocytes depends on Gβγ activation. Treatment of a range of cell types with gallein, a pharmacological inhibitor of Gβγ signalling, did not result in a reduction in CCL3 induced cellular migration, but resulted in enhanced calcium mobilisation following chemokine stimulation. Inhibition of PI3 kinase (PI3K) and AKT, which are activated downstream of Gβγ, equally had no effect on calcium release and a minor effect on cell migration. Similarly, inhibition of ERK1/2 did not prevent CCL3 induced migration. Interestingly, Gβγ as well as PI3K activation is necessary for CXCL11 induced migration of activated T-cells. These data not only confirm a role for Gβγ signalling in CXCL11 induced migration, but also demonstrate that targeting Gβγ as a therapeutic target to prevent migration in inflammatory disease may not be beneficial, at least not for CCL3 induced migration. This highlights the distinct differences in the mechanisms on how CC- and CXC-receptors activate cellular migration.