Structural and Functional Diversity of Acidic Scorpion Potassium Channel Toxins

Background: Although the basic scorpion K + channel toxins (KTxs) are well-known pharmacological tools and potential drug candidates, characterization the acidic KTxs still has the great significance for their potential selectivity towards different K + channel subtypes. Unfortunately, research on the acidic KTxs has been ignored for several years and progressed slowly. Principal Findings: Here, we describe the identification of nine new acidic KTxs by cDNA cloning and bioinformatic analyses. Seven of these toxins belong to three new a-KTx subfamilies (a-KTx28, a-KTx29, and a-KTx30), and two are new members of the known k-KTx2 subfamily. ImKTx104 containing three disulfide bridges, the first member of the a-KTx28 subfamily, has a low sequence homology with other known KTxs, and its NMR structure suggests ImKTx104 adopts a modified cystine-stabilized a-helix-loop-b-sheet (CS-a/b) fold motif that has no apparent a-helixs and b-sheets, but still stabilized by three disulfide bridges. These newly described acidic KTxs exhibit differential pharmacological effects on potassium channels. Acidic scorpion toxin ImKTx104 was the first peptide inhibitor found to affect KCNQ1 channel, which is insensitive to the basic KTxs and is strongly associated with human cardiac abnormalities. ImKTx104 selectively inhibited KCNQ1 channel with a Kd of 11.69 mM, but was less effective against the basic KTxs-sensitive potassium channels. In addition to the ImKTx104 toxin, HeTx204 peptide, containing a cystine-stabilized a-helix-loop-helix (CS-a/a) fold scaffold motif

Both core and terminal glycosylation alter epitope expression in thyrotropin and introduce discordances in hormone measurements

Thyroid-stimulating hormone (TSH) is routinely measured in blood to diagnose thyroid disorders using immunoassays. This study used recombinant TSH (recTSH) as a source of hormonal compound exhibiting a serum-type glycosylation and putatively reflecting physiopathological alterations in TSH polymorphism. Mass spectrometry revealed that in rec- TSH, both subunits display high-molecular-size glycoforms compared to the pituitary hormone (pitTSH), indicating more complex glycosylation. To determine how changes in TSH glycosylation may affect epitope expression, comparative epitope mapping of rec- and pitTSH was carried out using a panel of ten hormonespecific monoclonal antibodies. Three common epitopes, I, II and III, were identified as common to both preparations and allowed the design of six assays as I/II, II/I, I/III, III/I, II/III, and III/II. Highly sialylated rec- TSHs were produced by enzymatic remodeling to mimic the hormone circulating in blood and revealed limited expression of epitope I, but enhanced recognition of epitope II. Fractionation on a lentil lectin-Sepharose column allowed selection of nonfucosylated recTSH, thought to be associated with primary hypothyroidism. Recognition of epitope I was not modified by TSH core fucosylation, while epitope III expression was increased in non-fucosylated glycoforms. Taken together, our findings demonstrate that changes in both core and terminal glycosylation alter epitope expression in TSH and thereby induce highly variable antibody recognition, resulting in significant discordances among hormone measurements

Tamapin, a venom peptide from the Indian red scorpion (mesobuthus tamulus) that targets small conductance Ca2+-activated K+ channels and afterhyperpolarization currents in central neurons

The biophysical properties of small conductance Ca2+-activated K+ (SK) channels are well suited to underlie afterhyperpolarizations (AHPs) shaping the firing patterns of a conspicuous number of central and peripheral neurons. We have identified a new scorpion toxin (tamapin) that binds to SK channels with high affinity and inhibits SK channel-mediated currents in pyramidal neurons of the hippocampus as well as in cell lines expressing distinct SK channel subunits. This toxin distinguished between the SK channels underlying the apamin-sensitive IAHP and the Ca2+-activated K+ channels mediating the slow IAHP (sIAHP) in hippocampal neurons. Compared with related scorpion toxins, tamapin displayed a unique, remarkable selectivity for SK2 versus SK1 (~1750-fold) and SK3 (~70-fold) channels and is the most potent SK2 channel blocker characterized so far (IC50 for SK2 channels = 24 pM). Tamapin will facilitate the characterization of the subunit composition of native SK channels and help determine their involvement in electrical and biochemical signaling.</p