Identification and characterization of a novel monoamine transporter in the human brain

Precise control of monoamine neurotransmitter levels in the extracellular fluids of the brain is critical in maintaining efficient and robust neurotransmission. High affinity transporters in the solute carrier SLC6A family function in removing monoamines from the neurosynaptic cleft. Emerging evidence suggests that these transporters are only one part of a system of transporters that work in concert to maintain brain homeostasis of monoamines. Here we report the cloning and characterization of a new human plasma membrane monoamine transporter, PMAT. The PMAT cDNA encodes a protein of 530 amino acid residues with 10-12 transmembrane segments. PMAT is not homologous to known neurotransmitter transporters but exhibits low homology to members of the equilibrative nucleoside transporter family. When expressed in Madin-Darby canine kidney cells and Xenopus laevis oocytes, PMAT efficiently transports serotonin (K(m) = 114 mum), dopamine (K(m) = 329 mum), and the neurotoxin 1-methyl-4-phenylpyridinium (K(m) = 33 mum). In contrast, there is no significant interaction of PMAT with nucleosides or nucleobases. PMAT-mediated monoamine transport does not require Na(+) or Cl(-) but appears to be sensitive to changes in membrane potential. Northern blot analysis showed that PMAT is predominantly expressed in the human brain and widely distributed in the central nervous system. These studies demonstrate that PMAT may be a novel low affinity transporter for biogenic amines, which, under certain conditions, might supplement the role of the high affinity transporters in the brain.     

Ischnomelissa,a New Genus of Augochlorine Bees (Halictidae) from Colombia

A new augochlorine bee genus, Ischnomelissa gen. n., is described and figured. The genus is based on a new species from the mountains of Colombia, Ischnomelissa zonata. Ischnomelissa superficially resembles species of Caenaugochlora (Ctenaugochlora), but differs in having a rounded preoccipital ridge, an acute marginal cell apex, an elongated propodeum, and a single gonostylus (not divided into ventral and dorsal gonostylar processes). Color variation within I. zonata and the position of the group in augochlorine phylogeny is briefly discussed. Modified couplets are provided for Eickwort’s 1969 key to genera and subgenera in order to correctly identify the genus.     

Generation and Characterization of Yeast Two-Hybrid cDNA Libraries Derived From Two Distinct Mouse Pluripotent Cell Types

Pluripotent stem cells have the therapeutic potential in future regenerative medicine applications. Therefore, it is highly important to understand the molecular mechanisms governing the pluripotency and differentiation potential of these cells. Our current knowledge of pluripotent cells is largely limited owing to the candidate gene/protein approach rather than studying the complex interactions of the proteins. Experimentally, yeast two-hybrid system (Y2H) is by far the most useful and widely used method to detect the protein–protein interactions in high-throughput screenings. Unfortunately, currently there is no GAL4-based pluripotent stem cell-specific cDNA library available for screening the interaction proteins impeding the large-scale studies. In this study, we report the construction of Y2H cDNA libraries derived from mouse pluripotent embryonic stem cells (ESCs) and multipotent adult germ-line stem cells (maGSCs) in GAL4-based Y2H vector system with very high transformation efficiency. Furthermore, we have constructed two different baits and screened for interaction partners in an effort to characterize the libraries and also as a part of our ongoing studies. Consequently, many putative interaction proteins were identified in both cases and their interaction was further validated by direct-Y2H. The observed interactions between bait proteins and their respective analyzed putative interaction proteins were further confirmed using two independent approaches in mammalian cells, thus highlighting the biological significance of the identified interactor (s). Finally, we would like to make these cDNA libraries as a resource that can be distributed to the research community.     

Interaction of agrin with laminin requires a coiled-coil conformation of the agrin-binding site within the laminin gamma1 chain

Coiled-coil domains are found in a wide variety of proteins, where they typically specify subunit oligomerization. Recently, we have demonstrated that agrin, a multidomain heparan sulfate proteoglycan with a crucial role in the development of the nerve-muscle synapse, binds to the three-stranded coiled-coil domain of laminin-1. The interaction with laminin mediates the integration of agrin into basement membranes. Here we characterize the binding site within the laminin-1 coiled coil in detail. Binding assays with individual laminin-1 full-length chains and fragments revealed that agrin specifically interacts with the gamma1 subunit of laminin-1, whereas no binding to alpha1 and beta1 chains was detected. By using recombinant gamma1 chain fragments, we mapped the binding site to a sequence of 20 residues. Furthermore, we demonstrate that a coiled-coil conformation of this binding site is required for its interaction with agrin. The finding that recombinant gamma1 fragments bound at least 10-fold less than native laminin-1 indicates that the structure of the three-stranded coiled-coil domain of laminin is required for high-affinity agrin binding. Interestingly, no binding to a chimeric gamma2 fragment was observed, indicating that the interaction of agrin with laminin is isoform specific.     

Mapping of the laminin-binding site of the N-terminal agrin domain (NtA)

Agrin is a key organizer of acetylcholine receptor (AChR) clustering at the neuromuscular junction. The binding of agrin to laminin is required for its localization to synaptic basal lamina and other basement membranes. The high-affinity interaction with the coiled-coil domain of laminin is mediated by the N-terminal domain of agrin. We have adopted a structurally guided site-directed mutagenesis approach to map the laminin-binding site of NtA. Mutations of L117 and V124 in the C-terminal helix 3 showed that they are crucial for binding. Both residues are located in helix 3 and face the groove between the beta-barrel and the C-terminal helical segment of NtA. Remarkably, the distance between both residues matches a heptad repeat distance of two aliphatic residues which are solvent exposed in the coiled-coil domain of laminin. A lower but significant contribution originates from R43 and a charged cluster (E23, E24 and R40) at the open face of the beta-barrel structure. We propose that surface-exposed, conserved residues of the laminin gamma1 chain interact with NtA via hydrophobic and ionic interactions.