Acid Sequence Changes Research Articles

Cross-neutralization of two different trypanosome populations derived from a single organism.

Antigenic variation observed during the course of infections with Trypanosoma brucei in mammals is caused by amino acid sequence changes in a variable surface glycoprotein (VSG)1–3. Antisera to isolated VSGs generally react only with the surface of homologous trypanosomes in neutralization and immunofluorescence tests1,4, except in the case of certain ‘isotypic’ populations derived from different field isolates of trypanosomes5. An important question in evaluating the possibility of a vaccine is whether a single trypanosome can change into a second population which shares some, but not all, surface exposed epitopes during the limited time span of several sequential infections. If any trypanosomes in a mixture share exposed epitopes, the complexity of the necessary immunogen is reduced. We show here that two trypanosome clones WaTat 1.1 and WaTat 1.12, derived by differentiation from a single organism, have partially homologous VSGs and indeed share some similar exposed epitopes. Hence, a monospecific anti-VSG antiserum neutralizes both trypanosomes. However, several surface-exposed epitopes in WaTat 1.12 VSG exhibit reduced binding affinity for monoclonal antibodies made against WaTat 1.1 VSG and one epitope present on WaTat 1.1 trypanosomes is missing from the surface of WaTat 1.12 organisms.

Amino acid sequence changes in antigenic variants of type A influenza virus N2 neuraminidase

Pronase-released neuraminidase heads from six strains of influenza type A virus (of the H2N2 and H3N2 subtypes) isolated between 1957 and 1975 were examined for changes in amino acid sequence. In 469 residues 19 changes which occurred during this period were located. Two regions of the neuraminidase molecule were not examined, residues 188–210 which were insoluble, and residues 1–73 (or 76) which formed part of the stalk of the neuraminidase and were removed during Pronase digestion. Neuraminidase heads from seven variants of A/Tokyo/3/67 virus, selected with different monoclonal antibodies to the neuraminidase were also examined for sequence changes. In four of these, a single sequence change at position 344 of arginine to isoleucine was found, in another variant an asparagine residue at position 221 changed to histidine and in another the lysine at position 368 changed to glutamic acid. This last change also occurred in the field strains isolated in 1972 and 1975. In the seventh monoclonal variant the change could not be found and may be in the insoluble region. Some of these sequence changes were clustered into two highly variable regions in the neuraminidase comprising residues 344–347 and 367–370, suggesting that these regions may be involved in antigenic sites on the neuraminidase molecule. An antigenic map of N2 neuraminidase [R. G. Webster, V. S. Hinshaw, and W. G. Laver, Virology 117, 93–104 (1982)] suggested three or four nonoverlapping antigenic areas and the variants with changes at residues 344 and 368 were grouped in one of these antigenic regions. Sequence changes in the region of 344–347 were also associated with changes in the stability of the neuraminidase. The four monoclonal variants with a sequence change of Arg (344) to Ile possessed neuraminidase molecules which were destroyed by Pronase at 37° (but not at 20°) whereas wild-type neuraminidase was stable during digestion at 37°.

Neuraminidase gene from the early Asian strain of human influenza virus, A/RI/5-/57 (H2N2).

The complete structure of the neuraminidase gene from the A/RI/5-/57 strain of influenza virus has been determined. It is 1467 nucleotides long and codes for a protein of 469 amino acid residues. Comparison with the gene sequence for the N1 strains A/WSN/33 and A/PR/8/34, the N2 strain A/Udorn/72 and the protein sequence for the N2 strain A/Tokyo/3/67 shows the amino acid sequence changes that have occurred during antigenic shift (60%) and drift (7-9%).

Mechanism of antigenic drift in influenza virus: Amino acid sequence changes in an antigenically active region of Hong Kong (H3N2) influenza virus hemagglutinin

During antigenic drift in influenza viruses, changes in antigenicity are associated with changes in amino acid sequence of the large hemagglutinin polypeptide, HA1. In ten variants of Hong Kong (H3N2) influenza virus selected with monoclonal antibodies, the proline residue at position 143 in HA1 changed to serine, threonine, leucine or histidine. In other variants, asparagine 133 changed to lysine, glycine 144 to aspartic acid and serine 145 to lysine. All these changes are possible by single base changes in the RNA except the last, which requires a double base change. Residues 142 to 146 also changed in field strains of Hong Kong influenza isolated between 1968 and 1977 (Laver et al., 1980). The single amino acid sequence changes in HA1 of the monoclonal variants were detected by comparing the compositions of the soluble tryptic peptides from the variants with the known sequences of these peptides from wild-type virus. Two insoluble tryptic peptides, comprising residues 110 to 140 and 230 to 255 in the HA1 molecule, were not examined and we do not know if additional changes occurred in these regions. In order to determine whether sequential changes at the same position occurred during antigenic drift, antibody prepared against the new antigenic site on the variants in which proline 143 changed to histidine or threonine was used to select second generation variants of these variants. In the first case, the glycine residue (144) next to the histidine changed to aspartic acid, and in the second, the threonine residue at position 143 reverted to proline and the virus regained the antigenicity of wild-type. Although monoclonal antibodies revealed dramatic antigenic differences between the variants and wild-type virus, only those variants with changes at position 144 of glycine to aspartic acid or at position 145 of serine to lysine could be distinguished from wild-type virus using heterogeneous rabbit or ferret antisera. The other variants, including those which showed sequence changes in widely separated positions of HA1, could not be distinguished from wild-type with heterogeneous antisera. These findings suggest that sequence changes in the region comprising residues 142 to 146 of HA1 affect an important antigenic site on the hemagglutinin molecule, but how these changes affect the antigenic properties, or whether this region actually forms part of the antigenic site is not known.

Error propagation in intracellular information transfer

The translation of genetic information from polynucleotides to proteins is mediated by proteins themselves. The cyclic nature of this process admits the possibility of a feedback of errors which may become lethal to the cell. During ageing, it is known that cells in some organisms show increased levels of altered or defective protein, and it has been suggested that the propagation of macromolecular errors may play a causative role in the progressive loss of homeostasis with increasing age. Experimental studies of this hypothesis have so far been inconclusive, and it is shown that theoretical models of intracellular error propagation throw important light on the determinants of stability within the translation apparatus and can improve the design of future experiments, as well as aid in their interpretation. Critical features of any model are its assumptions about the amino acid sequence changes required for a component of the translation apparatus to become error-prone and about the magnitude of any resultant change in activity. Existing models, which differ in these respects, are critically compared, and one is shown to be more flexible than the rest. In common with others, this model predicts that a normally stable translation apparatus has a threshold error level above which stability cannot be regained. The risk of crossing onto an irreversible path to cell death is determined by the distance between the stable and threshold error levels, and experiments to estimate this “safety margin” are suggested. Evolutionary modification of translational stability is also discussed.

Amino acid sequence changes in the haemagglutinin of A/Hong Kong (H3N2) influenza virus during the period 1968--77.

Haemagglutinin molecules from nine strains of A/Hong Kong/68 (H3N2) influenza virus, isolated between 1968 and 1977, were examined for changes in amino acid sequences. At least 18 changes, 9 of which were located precisely, occurred in the soluble tryptic peptides of the large haemagglutinin polypeptide (HA1) during this period. These peptides contained 262 residues (82% of HA1). In HA2, only two changes in 129 residues (58% of HA2) were detected. Sequential changes at a particular locus were not found; and as far as we can tell, once an amino acid changed, it did not change again in any subsequent variant examined.

Amino acid sequence change associated with genetic marker Inv(2) of human immunoglobulin.

IMMUNOGLOBULINS are proteins composed of two heavy and two light chains covalently linked by interchain disulphide bonds1. There are two types of light chains, κ and λ. The κ type human light chain is associated with an antigenic marker called Inv (ref. 2). Three types of antigens are recognised namely Inv (1), Inv(2) and Inv(3). Ropartz, Rivat and Rousseau3 have shown that these Inv markers are inherited through a series of three alleles Inv1,2, Inv1 and Inv3. About 98% of Caucasoid individuals who have the Inv(1) antigen also have the Inv (2) antigen3. Since fewer than 20% Cauca-soids have the Inv(1, −2) phenotype4 it follows that the Inv1 allele is rare in this race.

Identification and Mutational Relocation of the AUG Codon Initiating Translation of Iso-1-Cytochrome c in Yeast

Abstract Mutational studies and amino acid sequence analysis were used to identify the initiating methionyl codon AUG as an essential element at the beginning of the genetic message for iso-1-cytochrome c in yeast. Nine cy1 mutants completely lack iso-1-cytochrome c as a result of mutations at one end of the CY1 gene. Amino acid sequence changes of iso-1-cytochrome c from 45 intragenic revertants of these mutants were determined. No sequence changes occurred in 21 of the revertants and the changes in the rest were additions or deletions at the NH2 terminus. The additions were either Met-Ile-, Met-Leu-, Met-Arg-, or in one case, both Met- Val- and Val-. The additions were all alike in the revertants of any cy1 mutant. The deletions were always losses of the first four residues of the protein, Thr-Glu-Phe-Lys-. Normal amounts of iso-1-cytochrome c occurred in all of the revertants except those containing the short protein, which contained only 51% of the normal amount. The nine cy1 mutants had incurred single base pair substitutions at the three nucleotides immediately preceding the codon for the NH2-terminal threonine. The essential, initial triplet is the methionyl codon, AUG. Its conversion to codons for isoleucine, leucine, arginine, or valine prevents the formation of the protein. Reversions either reverse the original substitutions, leading to the normal protein, or generate new methionine codons at the left of the original AUG position, producing the long proteins, or they convert the normal lysine 4 codon to AUG, creating the short protein. A methionine aminopeptidase is proposed to exist, which cleaves methionine efficiently from Met-Thr- and Met-Ala- bonds, less efficiently from Met-Val- bonds, and not at all from Met-Leu, Met-Ile-, and Met-Arg- bonds. The cy1 initiator mutants were used to demonstrate that ethyl methanesulfonate induces the transition G·C → A·T at a frequency at least 100 times greater than it causes the transversions A·T → T·A and G·C → T·A and one or more unidentified base pair exchanges to G·C. The function of the essential AUG triplet is presumed to be initiation of translation. This function is not provided by at least 35 of the 64 nucleotide triplets; notably it is not provided by GUG. The amount of iso-1-cytochrome c in yeast is under the control of the rate of initiation of translation, which may be reduced or remain unaltered following mutational relocation of the initiator codon.