Of cytokine receptor scissors, grasping bacterial hands, and movement across the blood–brain barrier

This month's installment of Generally Physiological focuses on movement across membranes, discussing how the human growth hormone (hGH) receptor transmits the ligand-binding signal across the cell membrane to activate intracellular signaling pathways , how the bacterial outer membrane transport protein FepA reaches out to grasp its substrate, and the identification of two disparate functions for an orphan transporter at the blood– brain barrier. The human growth hormone (hGH) receptor functions as a dimer: a single molecule of the hGH poly peptide binds sequentially to the extracellular domains of iden tical single-trans-membrane domain monomeric sub-units to activate a pair of Janus kinase-2 (JAK2) protein kinases associated with the intracellular domains. The mechanism whereby hGH binding the receptor extracellular domains initiates the intracellular JAK-STAT signaling pathway, however, has been unclear. Early models postulating that hGH binding drew together the two monomers, and thereby their in-tracellular domains, were cast into doubt by data indicating that the un-liganded receptor exists as a dimer (see Wells and Kossiakoff, 2014). Using a combination of Förster resonance energy transfer (FRET) analysis, cysteine cross-linking, mutagenesis, and molecular dynamic simulations, Brooks et al. (2014) developed a " scissor-like " molecular model for JAK2 activation in which the two JAK2 molecules associated with a resting hGH receptor dimer are oriented so as to mutually inhibit each other through pseudokinase domains. hGH binding elicits a confor-mational change in the receptor that results in the separation of the membrane -proximal JAK2-binding regions in the receptor intracellular domain; this leads to the movement apart of the JAK2 molecules, relieving their mutual inhibition and juxtaposing their kinase domains, enabling them to activate each other and initiate the downstream signal-ing pathway. Thus, in marked contrast to the notion that hGH binding draws together the receptor intracel-lular domains, it appears to promote their separation. Gram-negative bacteria like Esche-richia coli accumulate the essential micronutrient iron bound to sider-ophores such as ferric enterobactin (FeEnt), transporting the complex through the selectively permeable bacterial outer membrane by means of transporters such as FepA, a 22-stranded transmembrane -barrel protein. Ligand transport by FepA depends on an initial step of high-affinity binding by surface loops, followed by a second step involving energy-dependent interactions with an additional protein, TonB; both of these steps depend on conform-ational changes in FepA. In this issue, Smallwood et al. used site-directed spectroscopic analysis of fluor ophore-labeled E. coli FepA to characterize the movement of seven individual surface …