Nuclear magnetic resonance studies of spherical plant viruses

Two histidines of the coat protein of turnip yellow mosaic virus at the capsid interior are crucial for viability

RNA-coat protein interactions in turnip yellow mosaic virus (TYMV) have been shown to involve low pK proton-donating groups. Two different types of interaction have been proposed. In the so-called type I interaction, protonated C-residues interact with acidic amino acids at low pH, thereby providing a rationale for the high C-content (38%) of the genomic RNA. The type II interaction involves charged histidines interacting with phosphates of the RNA backbone. Site-directed mutagenesis of the TYMV coat protein and subsequent in vivo analysis were performed to distinguish between these two types of RNA-protein interaction. The results reveal a prominent role for the histidines H68 and H180, since mutation to an alanine residue inhibits symptom development on secondary leaves, indicating that spreading of the virus in the plant is blocked. Viral RNA and coat protein synthesis are not altered, showing that these two histidines may play a role in the process of RNA encapsidation. Overexpression of the TYMV coat protein in Escherichia coli leads to the formation of bona fide capsids, showing that the two histidines are not critical in capsid assembly. Mutagenesis of the acidic amino acids D11, E135, and D143 to alanine apparently did not interfere with virus viability. The functional role of the histidines during the infection cycle is discussed in terms of the structure of the coat protein, both at the level of amino acid sequence conservation among the members of the Tymoviridae family and as the three-dimensional structure of the coat protein.

A functional role for the conserved protonatable hairpins in the 5' untranslated region of turnip yellow mosaic virus RNA

The 5' untranslated region (UTR) of the RNA of several tymoviruses contains conserved hairpins with protonatable internal loops, consisting of C-C and C-A mismatches (K. Hellendoorn, P. J. A. Michiels, R. Buitenhuis, and C. W. A. Pleij, Nucleic Acids Res. 24, 4910-4917, 1996). Here, we present a functional analysis of the 5' UTR of turnip yellow mosaic virus (TYMV) RNA, which contains two protonatable hairpins with nearly identical internal loops. Mutations were introduced in an infectious cDNA clone, and T7 RNA transcripts were used to infect Chinese cabbage plants. Different symptoms were observed for the various mutants, pointing to a functional role of the C-C and C-A mismatches in the hairpins of the 5' UTR. The replication of the virus is influenced by the mutations made, while in vitro translation studies showed that the expression of the two overlapping reading frames of TYMV is not influenced by the secondary structure of the leader. Various mutants were propagated for up to five serial passages of infection, and the sequence of the 5' UTR was determined. This resulted in virus RNA with new non-wild-type sequences that produced the wild-type phenotype in infected plants. Remarkably, in all cases C-C or C-A mismatches were introduced. The internal loop of the 5'-proximal hairpin seems to be more important for the viral life cycle than that of the second hairpin. A deletion of 75% of the leader, including the two hairpins, resulted in a virus that was deficient in viral spread. Since the ratio between filled and empty capsids was changed drastically by this mutation, a role of the 5' UTR in viral packaging is proposed.

A cyclic nucleotide-gated conductance in olfactory receptor cilia

Olfactory transduction is thought to be initiated by the binding of odorants to specific receptor proteins in the cilia of olfactory receptor cells. The mechanism by which odorant binding could initiate membrane depolarization is unknown, but the recent discovery of an odorant-stimulated adenylate cyclase in purified olfactory cilia suggests that cyclic AMP may serve as an intracellular messenger for olfactory transduction. If so, then there might be a conductance in the ciliary plasma membrane which is controlled by cAMP. Here we report that excised patches of ciliary plasma membrane, obtained from dissociated receptor cells, contain a conductance which is gated directly by cAMP. This conductance resembles the cyclic GMP-gated conductance that mediates phototransduction in rod and cone outer segments, but differs in that it is activated by both cAMP and cGMP. Our data provide a mechanistic basis by which an odorant-stimulated increase in cyclic nucleotide concentration could lead to an increase in membrane conductance and therefore, to membrane depolarization. These data suggest a remarkable similarity between the mechanisms of olfactory and visual transduction and indicate considerable conservation of sensory transduction mechanisms.

Ionic channels: II. Voltage- and agonist-gated and agonist-modified channel properties and structure

This article reviews the different forms of ionic channels: voltage-gated, agonist-gated, and agonist- and second messenger-modified channels. The recent advances in our knowledge of the amino acid sequence of the sodium channel and the nicotinic acetylcholine receptor and the relationship of the primary structure to the channels' quarternary structure and function are discussed.

Ionic channels: I. The biophysical basis for ion passage and channel gating

This article reviews the biophysics of ion passage through membrane pores, as well as the physical factors that control the ion selectivity, gating, and conductance of an ionic channel. Different voltage clamp techniques are discussed in detail. The biophysical properties of sodium channels are reviewed.

Single cyclic GMP-activated channel activity in excised patches of rod outer segment membrane

The plasma membrane of retinal rod outer segments contains a cyclic GMP-activated conductance which appears to be the light-sensitive conductance involved in phototransduction. Recently, it has been found that this conductance is partially blocked by Mg2+ and Ca2+ at physiological concentrations, thus possibly accounting for the absence of observable single-channel activity in excised membrane patches and for the unusually small apparent unit conductance deduced from noise measurements on intact cells. We now report that, as expected from this idea, single cGMP-activated channel activity can be detected from an excised rod membrane patch in the absence of divalent cations. The most prominent unitary current had a mean conductance of approximately 25 pS. Both individual channel openings (mean open time approximately 1 ms) and short bursts of openings (mean burst duration of about a few milliseconds) were observed. In addition, there were smaller events which probably represented other states of the conductance. The mean current increased with the third power of cGMP concentration, suggesting that there are at least three cGMP-binding sites on the channel molecule. With 0.2 mM Mg2+ in the cGMP-containing solution, a flickering block of the open channel was observed; the effect of Ca2+ was similar. The results resolve a puzzle about the light-sensitive conductance by demonstrating that it is an aqueous pore rather than a carrier.