Amino acid intrinsic alpha-helical propensities III: positional dependence at several positions of C terminus

In this study, we have analyzed experimentally the helical intrinsic propensities of non-charged and non-aromatic residues at different C-terminal positions (C1, C2, C3) of an Ala-based peptide. The effect was found to be complex, resulting in extra stabilization or destabilization, depending on guest amino acid and position under consideration. Polar (Ser, Thr, Cys, Asn, and Gln) amino acids and Gly were found to have significantly larger helical propensities at several C-terminal positions compared with the alpha-helix center (-1.0 kcal/mole in some cases). Some of the nonpolar residues, especially beta-branched ones (Val and Ile) are significantly more favorable at position C3 (-0.3 to -0.4 kcal/mole), although having minor differences at other C-terminal positions compared with the alpha-helix center. Leu has moderate (-0.1 to -0.2 kcal/mole) stabilization effects at position C2 and C3, whereas being relatively neutral at C1. Finally, Met was found to be unfavorable at C1 and C2 ( +0.2 kcal/mole) and favorable at C3 (-0.2 kcal/mole). Thus, significant differences found between the intrinsic helical propensities at the C-terminal positions and those in the alpha-helix center must be accounted for in helix/coil transition theories and in protein design.

The C terminus of the Ca channel alpha1B subunit mediates selective inhibition by G-protein-coupled receptors

Inhibition of calcium channels by G-protein-coupled receptors depends on the nature of the Galpha subunit, although the Gbetagamma complex is thought to be responsible for channel inhibition. Ca currents in hypothalamic neurons and N-type calcium channels expressed in HEK-293 cells showed robust inhibition by G(i)/G(o)-coupled galanin receptors (GalR1), but not by Gq-coupled galanin receptors (GalR2). However, deletions in the C terminus of alpha(1B-1) produced Ca channels that were inhibited after activation of both GalR1 and GalR2. Inhibition of protein kinase C (PKC) also revealed Ca current modulation by GalR2. Imaging studies using green fluorescent protein fusions of the C terminus of alpha(1B) demonstrated that activation of the GalR2 receptor caused translocation of the C terminus of alpha(1B-1) to the membrane and co-localization with Galphaq and PKC. Similar translocation was not seen with a C-terminal truncated splice variant, alpha(1B-2). Immunoprecipitation experiments demonstrated that Galphaq interacts directly with the C terminus of the alpha(1B) subunit. These results are consistent with a model in which local activation of PKC by channel-associated Galphaq blocks modulation of the channel by Gbetagamma released by Gq-coupled receptors.

C-Terminal alternative splicing changes the gating properties of a human spinal cord calcium channel alpha 1A subunit

The calcium channel alpha(1A) subunit gene codes for proteins with diverse structure and function. This diversity may be important for fine tuning neurotransmitter release at central and peripheral synapses. The alpha(1A) C terminus, which serves a critical role in processing information from intracellular signaling molecules, is capable of undergoing extensive alternative splicing. The purpose of this study was to determine the extent to which C-terminal alternative splicing affects some of the fundamental biophysical properties of alpha(1A) subunits. Specifically, the biophysical properties of two alternatively spliced alpha(1A) subunits were compared. One variant was identical to an isoform identified previously in human brain, and the other was a novel isoform isolated from human spinal cord. The variants differed by two amino acids (NP) in the extracellular linker between transmembrane segments IVS3 and IVS4 and in two C-terminal regions encoded by exons 37 and 44. Expression in Xenopus oocytes demonstrated that the two variants were similar with respect to current-voltage relationships and the voltage dependence of steady-state activation and inactivation. However, the rates of activation, inactivation, deactivation, and recovery from inactivation were all significantly slower for the spinal cord variant. A chimeric strategy demonstrated that the inclusion of the sequence encoded by exon 44 specifically affects the rate of inactivation. These findings demonstrate that C-terminal structural changes alone can influence the way in which alpha(1A) subunits respond to a depolarizing stimulus and add to the developing picture of the C terminus as a critical domain in the regulation of Ca(2+) channel function.

C terminus of presenilin is required for overproduction of amyloidogenic Abeta42 through stabilization and endoproteolysis of presenilin

Mutations in presenilin (PS) genes cause early onset familial Alzheimer's disease (FAD) by increasing production of the amyloidogenic form of amyloid beta peptides ending at residue 42 (Abeta42). To identify a PS subdomain responsible for overproduction of Abeta42, we analyzed neuro2a cell lines expressing modified forms of PS2 that harbor an N141I FAD mutation. Deletion or addition of amino acids at the C terminus and Ile448 substitution in PS2 with the N141I FAD mutation abrogated the increase in Abeta42 secretion, and Abeta42 overproduction was dependent on the stabilization and endoproteolysis of PS2. The same C-terminal modifications in PS1 produced similar effects. Hence, we suggest that the C terminus of PS plays a crucial role in the overproduction of Abeta42 through stabilization of endoproteolytic PS derivatives and that these derivatives may be the pathologically active species of PS that cause FAD.

Splice variant-specific interaction of the NMDA receptor subunit NR1 with neuronal intermediate filaments

NMDA receptors are excitatory neurotransmitter receptors critical for synaptic plasticity and neuronal development in the mammalian brain. These receptors are found highly concentrated in the postsynaptic membrane of glutamatergic synapses. To investigate the molecular mechanisms underlying NMDA receptor localization, we used the yeast two-hybrid system to identify proteins expressed in the brain that interact with the NMDA receptor subunit NR1. Here we report that the 68 kDa neurofilament subunit NF-L directly interacts with the NR1 subunit. This interaction occurs between the cytoplasmic C-terminal domain of NR1 and the rod domain of NF-L. However, NR1 splice variants lacking the first C-terminal exon cassette (C1) failed to associate with NF-L. Immunogold electron microscopy revealed a preferential localization of NR1 at the ends of in vitro-assembled neurofilaments. Overexpression of C1 cassette-containing NR1 constructs in fibroblast cells disrupted the assembly of recombinant neurofilaments. In addition, NR1 and NF-L cofractionated in detergent-treated rat brain synaptic plasma membranes. Furthermore, NR1 and NF-L colocalize in the dendrites and growth cones of cultured hippocampal neurons. These results demonstrate the splice variant-specific association of NR1 with neurofilaments and suggest a possible mechanism for anchoring or localizing NMDA receptors in the neuronal plasma membrane.