A facile approach for the in vitro assembly of multimeric membrane transport proteins

Membrane proteins such as ion channels and transporters are frequently homomeric. The homomeric nature raises important questions regarding coupling between subunits and complicates the application of techniques such as FRET or DEER spectroscopy. These challenges can be overcome if the subunits of a homomeric protein can be independently modified for functional or spectroscopic studies. Here, we describe a general approach for in vitro assembly that can be used for the generation of heteromeric variants of homomeric membrane proteins. We establish the approach using Glt_Ph, a glutamate transporter homolog that is trimeric in the native state. We use heteromeric Glt_Ph transporters to directly demonstrate the lack of coupling in substrate binding and demonstrate how heteromeric transporters considerably simplify the application of DEER spectroscopy. Further, we demonstrate the general applicability of this approach by carrying out the in vitro assembly of VcINDY, a Na⁺-coupled succinate transporter and CLC-ec1, a Cl^-/H⁺ antiporter.

Combining in Vitro Folding with Cell Free Protein Synthesis for Membrane Protein Expression

Cell free protein synthesis (CFPS) has emerged as a promising methodology for protein expression. While polypeptide production is very reliable and efficient using CFPS, the correct cotranslational folding of membrane proteins during CFPS is still a challenge. In this contribution, we describe a two-step protocol in which the integral membrane protein is initially expressed by CFPS as a precipitate followed by an in vitro folding procedure using lipid vesicles for converting the protein precipitate to the correctly folded protein. We demonstrate the feasibility of using this approach for the K(+) channels KcsA and MVP and the amino acid transporter LeuT. We determine the crystal structure of the KcsA channel obtained by CFPS and in vitro folding to show the structural similarity to the cellular expressed KcsA channel and to establish the feasibility of using this two-step approach for membrane protein production for structural studies. Our studies show that the correct folding of these membrane proteins with complex topologies can take place in vitro without the involvement of the cellular machinery for membrane protein biogenesis. This indicates that the folding instructions for these complex membrane proteins are contained entirely within the protein sequence.

Engineering the glutamate transporter homologue GltPh using protein semisynthesis

Glutamate transporters catalyze the concentrative uptake of glutamate from synapses and are essential for normal synaptic function. Despite extensive investigations of glutamate transporters, the mechanisms underlying substrate recognition, ion selectivity, and the coupling of substrate and ion transport are not well-understood. Deciphering these mechanisms requires the ability to precisely engineer the transporter. In this study, we describe the semisynthesis of GltPh, an archaeal homologue of glutamate transporters. Semisynthesis allows the precise engineering of GltPh through the incorporation of unnatural amino acids and peptide backbone modifications. In the semisynthesis, the GltPh polypeptide is initially assembled from a recombinantly expressed thioester peptide and a chemically synthesized peptide using the native chemical ligation reaction followed by in vitro folding to the native state. We have developed a robust procedure for the in vitro folding of GltPh. Biochemical characterization of the semisynthetic GltPh indicates that it is similar to the native transporter. We used semisynthesis to substitute Arg397, a highly conserved residue in the substrate binding site, with the unnatural analogue, citrulline. Our studies demonstrate that Arg397 is required for high-affinity substrate binding, and on the basis of our results, we propose that Arg397 is involved in a Na+-dependent remodeling of the substrate binding site required for high-affinity Asp binding. We anticipate that the semisynthetic approach developed in this study will be extremely useful in investigating functional mechanisms in GltPh. Further, the approach developed in this study should also be applicable to other membrane transport proteins.

Studies of ion channels using expressed protein ligation

Expressed protein ligation (EPL) is a semisynthetic technique for the chemoselective ligation of a synthetic peptide to a recombinant peptide that results in a native peptide bond at the ligation site. EPL therefore allows us to engineer proteins with chemically defined, site-specific modifications. While EPL has been used mainly in investigations of soluble proteins, in recent years it has been increasingly used in investigations of integral membrane proteins. These include studies on the KcsA K(+) channel, the non-selective cation channel NaK, and the porin OmpF. These studies are discussed in this review.

GDNF and insulin cooperate to enhance the proliferation and differentiation of enteric crest-derived cells

Previously we have shown that glial derived neurotrophic factor (GDNF) stimulates modest increases in the proliferation of avian enteric crest-derived cells and similar increases in the phosphorylation of the phosphoinositide 3-kinase (PI3K) downstream substrate Akt (Akt-P). In the present study we tested whether GDNF-independent increases in PI3K activation would be sufficient to support proliferation. We found that insulin induces a large increase in the phosphorylation of Akt and can initiate DNA synthesis in avian enteric crest-derived cells, but is unable to maintain proliferation over time in culture, measured by BrdU incorporation. GDNF can also initiate DNA synthesis, but it too is unable to maintain BrdU incorporation in cultured enteric crest-derived cells. Sustained incorporation of BrdU after 16-48 h in culture is shown to be dependent on a combination of GDNF and insulin. Using a phospho-specific antibody, we found Akt-P levels to be similar in the proliferating (BrdU incorporation maintained from 16-48 h in culture) and nonproliferating populations, suggesting that Akt-P levels were not solely controlling the extent of BrdU incorporation. A minimum level of PI3K activation, however, is required, as shown by the dose-dependent reduction in proliferation with the PI3K inhibitor LY-294002. We conclude that the integrity of the PI3K pathway is essential for enteric crest-derived cell proliferation, but that the absolute levels of Akt-P do not determine the extent of proliferation. The enhanced proliferation in cultures containing both GDNF and insulin suggests that other pathways are involved, including the possibility that PI3K downstream effectors other than Akt are important in the regulation of avian enteric crest-derived cell proliferation.

Appearance of neurons and glia with respect to the wavefront during colonization of the avian gut by neural crest cells

The enteric nervous system is formed by neural crest cells that migrate, proliferate, and differentiate into neurons and glia distributed in ganglia along the gastrointestinal tract. In the developing embryo some enteric crest cells cease their caudal movements, whereas others continue to migrate. Subsequently, the enteric neurons form a reticular network of ganglia interconnected by axonal projections. We studied the developing avian gut to characterize the pattern of migration of the crest cells, and the relationship between migration and differentiation. Crest cells at the leading edge of the migratory front appear as strands of cells; isolated individual crest cells are rarely seen. In the foregut and midgut, these strands are located immediately beneath the serosa. In contrast, crest cells entering the colon appear first in the deeper submucosal mesenchyme and later beneath the serosa. As the neural crest wavefront passes caudally, the crest cell cords become highly branched, forming a reticular lattice that presages the mature organization of the enteric nervous system. Neurons and glia first appear within the strands at the advancing wavefront. Later neurons are consistently located at the nodes where branches of the lattice intersect. In the most rostral foregut and in the colon, some neurons initially appear in close association with extrinsic nerve fibers from the vagus and Remak's nerve, respectively. We conclude that crest cells colonize the gut as chains of cells and that, within these chains, both neurons and glia appear close to the wavefront.

Enteric neuroblasts require the phosphatidylinositol 3-kinase pathway for GDNF-stimulated proliferation

The enteric nervous system (ENS) develops from neural crest cells that enter the gut, migrate, proliferate, and differentiate into neurons and glia. The growth factor glial-derived neurotrophic factor (GDNF) stimulates the proliferation and survival of enteric crest-derived cells. We investigated the intracellular signaling pathways activated by GDNF and their involvement in proliferation. We found that GDNF stimulates the phosphorylation of both the PI 3-kinase downstream substrate Akt and the MAP kinase substrate ERK in cultures of immunoaffinity-purified embryonic avian enteric crest-derived cells. The selective PI 3-kinase inhibitor LY-294002 blocked GDNF-stimulated Akt phosphorylation in purified crest cells, and reduced proliferation in cultures of dissociated quail gut. The ERK kinase (MEK) inhibitors PD 98059 and UO126 did not reduce GDNF-stimulated proliferation, although PD 98059 blocked GDNF-stimulated phosphorylation of ERK. We conclude that the PI 3-kinase pathway is necessary for the GDNF-stimulated proliferation of enteric neuroblasts.