Glycosylation and surface expression of the influenza virus neuraminidase requires the N-terminal hydrophobic region

The amino-terminal region of the neuraminidase protein from avian H5N1 influenza virus is important for its biosynthetic transport to the host cell surface

Influenza virus neuraminidase (NA) is a major viral envelope glycoprotein, which plays a critical role in viral infection. Although NA functional domains have been determined previously, the precise role of the amino acids located at the N-terminus of avian H5N1 NA for protein expression and intracellular transport to the host plasma membrane is not fully understood. In the present study, a series of N-terminal truncation or deletion mutants of H5N1 NA were generated and their expression and intracellular trafficking were investigated. Protein expression from mutants NAΔ20, NAΔ35, NAΔ40, NAΔ7-20 and NAΔ7-35 was undetectable by immunoblotting and by performing NA activity assays. Mutants NAΔ6, NAΔ11 and NAΔ15-20 showed a marked decreased in protein expression, whereas mutants NAΔ7-15 and NAΔ15 displayed a slight increase in protein expression, compared with that of the native NA protein. These data suggest that amino acid residues 16-20 are vital for NA protein expression, while amino acids 7-15 might suppress NA protein expression. In deletion mutants NAΔ7-15 and NAΔ15 there was an accumulation of NA protein at the juxta-nuclear region, with reduced expression of NA at the cell surface. Although active Cdc42 could promote transport of wild-type NA to the host cell surface, this member of the Rho family of GTPases failed to regulate transport of mutants NAΔ7-15 and NAΔ15. The results of the study reveal that amino acid residues 7-15 of H5N1 NA are critical for its biosynthetic transport to the host cell surface.

Homosubtypic and heterosubtypic antibodies against highly pathogenic avian influenza H5N1 recombinant proteins in H5N1 survivors and non-H5N1 subjects

Six recombinant vaccinia viruses containing HA, NA, NP, M or NS gene insert derived from a highly pathogenic avian influenza H5N1 virus, and the recombinant vaccinia virus harboring plasmid backbone as the virus control were constructed. The recombinant proteins were characterized for their expression and subcellular locations in TK(-) cells. Antibodies to the five recombinant proteins were detected in all 13 sequential serum samples collected from four H5N1 survivors during four years of follow-up; and those directed to rVac-H5 HA and rVac-NA proteins were found in higher titers than those directed to the internal proteins as revealed by indirect immunofluorescence assay. Although all 28 non-H5N1 subjects had no neutralizing antibodies against H5N1 virus, they did have cross-reactive antibodies to those five recombinant proteins. A significant increase in cross-reactive antibody titer to rVac-H5 HA and rVac-NA was found in paired blood samples from patients infected with the 2009 pandemic virus.

A conserved internal hydrophobic domain mediates the stable membrane integration of the dengue virus capsid protein

The mature flavivirus capsid protein (virion C) is commonly thought to be free in the cytoplasm of infected cells and to form a nucleocapsid-like complex with genomic RNA in mature virus particles. There is little sequence conservation among flavivirus virion C proteins, but they are similar in size (e.g., 99 amino acids [aa] for the dengue-4 [DEN4] C) and in bearing a net positive charge. In addition, we noted that C contained a conserved internal hydrophobic segment (spanning aa 45-65 in the DEN4 C). Results of in vivo expression and in vitro translation of wt and mutant forms of the DEN4 virion C demonstrated that the conserved internal hydrophobic segment in the DEN C functioned as a membrane anchor domain. Signal peptide function of this segment was also suggested by its requirement for the entry of C into membranes. Virion C was integrated in membranes in a "hairpin" conformation; positively charged segments amino- and carboxy-terminal to the hydrophobic signal-anchor segment were accessible to protease digestion in the "cytoplasm." The net positive charge in the amino-terminal extramembraneous portion of C (aa 1-44) was one determinant of the hairpin membrane orientation; a conserved positively charged residue within the hydrophobic segment (Arg-54 in the DEN4 C) was not.

GTP is required for the integration of a fragment of the Neurospora crassa H(+)-ATPase into homologous microsomal vesicles

The integration of a fragment of the Neurospora crassa plasma membrane H(+)-ATPase was examined to determine if insertion of the fragment into homologous microsomal vesicles is obligatorily dependent on a nucleoside triphosphate. RNA transcripts that encoded the amino terminal 344 amino acids of the Neurospora crassa plasma membrane H(+)-ATPase(pma(344)+) were translated in a N. crassa in vitro system. The pma(344)+ integrated post-translationally into homologous microsomal vesicles independent of the associated ribosomes and dependent on the presence of GTP or guanylyl imidodiphosphate, a nonhydrolyzable analogue of GTP. ATP or analogues thereof did not support the integration of pma(344)+ into nRM post-translationally. These results were interpreted to suggest that a GTPase plays an essential role in the integration of the amino terminal portion of the pma+ into the endoplasmic reticulum.

Internally located cleavable signal sequences direct the formation of Semliki Forest virus membrane proteins from a polyprotein precursor

The proteolytic processes involved in the cotranslational production of the Semliki Forest virus proteins p62, 6K, and E1 from a common precursor polypeptide were analyzed by an in vitro translation-translocation assay. By studying the behavior of wild-type and mutant variants of the polyprotein, we show that the signal sequences responsible for membrane translocation of the 6K and E1 proteins reside in the C-terminal regions of p62 and 6K, respectively. We present evidence suggesting that the polyprotein is processed on the luminal side by signal peptidase at consensus cleavage sites immediately following the signal sequences. Our results also lead us to conclude that the 6K protein is a transmembrane polypeptide with its N terminus on the luminal side of the membrane (type I). Thus, the production of all three membrane proteins is directed by alternating signal and stop-transfer (anchor) sequences that function in translocation and cleavage of the virus precursor polyprotein. This also shows conclusively that internally located signal sequences can be cleaved by signal peptidase.

Effect of mutations and deletions in a bicistronic mRNA on the synthesis of influenza B virus NB and NA glycoproteins

The mRNA derived from influenza B virus RNA segment 6 is functionally bicistronic and encodes the NB and NA glycoproteins in different, overlapping reading frames. NB protein synthesis is initiated at the 5'-proximal AUG codon, and 4 nucleotides downstream there is a second AUG codon which is used to initiate NA protein synthesis. The nucleotide sequence context of the first AUG codon conforms closely with the established 5'-CC(A/G)CCAUGG-3' consensus sequence (M. Kozak, Nucleic Acids Res. 15:8125-8148, 1987), which should favor initiation of NB protein synthesis at this site, yet NB and NA are found to accumulate in approximately equal amounts in infected cells. To determine the features important for allowing initiation at the second 5'-proximal AUG codon, we made changes in the 5'-terminal region of the mRNA, including deletions, insertions, and site-specific mutations. The recombinant DNA molecules were expressed in eucaryotic cells, and the accumulation of NB and NA was quantitated. The data indicate that changes in the immediate sequence around the first AUG codon do not make a large difference in the amounts of NB and NA that accumulate, but that when the first AUG codon is displaced from its normal position it is now quite efficient at preventing downstream initiation events. In addition, the data indicate that an element of the B/NB/NA mRNA 5' untranslated leader region acts in cis to enhance the expression of NB and NA.

Foreign transmembrane peptides replacing the internal signal sequence of transferrin receptor allow its translocation and membrane binding

Each subunit of the human transferrin receptor (TR) dimer is inserted into the ER membrane as a transmembrane polypeptide having its N-terminus in the cytoplasm. The transmembrane segment of the molecule serves both as a signal for chain translocation and as a membrane anchor. To study which structural features of this segment are required for its dual function, we have essentially replaced the transmembrane peptide with the C-terminal membrane-spanning segment of two proteins having a separate N-terminal translocation signal and with an artificial uncharged peptide. In each case the mutant TR molecules are efficiently translocated in vitro. In contrast, substitution of the transmembrane peptide of TR with a hydrophilic peptide results in no detectable translocation activity of the mutant TR. This suggests that the hydrophobic character of the transmembrane peptide of TR, rather than its actual amino acid sequence, is important for chain translocation and membrane binding.

Domains of virus glycoproteins

This chapter reviews current information about the structure and function of virus glycoproteins. There are few virus glycoproteins that provide prototypes for illustrating important relationships between the functions and glycoprotein structure. The discussion presented in the chapter concentrates on those viral glycoproteins that (1) span the lipid bilayer once, (2) are oriented such that the carboxy terminus comprises the cytoplasmic domain, and (3) contain asparagine-linked oligosaccharides. There are also viral glycoproteins with extensive O-linked glycosylation, some of which are also presented in the discussion. The chapter also focuses on the studies involving directed mutagenesis and construction of chimeric proteins. The effects of altering specific amino acid sequences, of swapping domains, and of adding a new domain to a protein serve to define the functions of a domain and to show that a domain can be independently associated with a specific function. The experiments described have been carried out by inserting the genes of particular viral glycoproteins—such as cDNAs—into expression vectors and transcribing the cDNAs from the promoter provided by the expression vector. This approach established that localization and functions such as the fusogenic activity are properties of the viral glycoprotein per se and do not require other viral-coded components.

The membrane-spanning segment of invariant chain (I gamma) contains a potentially cleavable signal sequence

The human invariant chain (I gamma) of class II histocompatibility antigens spans the membrane of the endoplasmic reticulum once. It exposes a small amino-terminal domain on the cytoplasmic side and a carboxy-terminal, glycosylated domain on the exoplasmic side of the membrane. When the exoplasmic domain of I gamma is replaced by the cytoplasmic protein chloramphenicol acetyltransferase (CAT), CAT becomes the exoplasmic, glycosylated domain of the resulting membrane protein I gamma CAT. Deletion of the hydrophilic cytoplasmic domain from I gamma CAT gives rise to a secreted protein from which an amino-terminal segment is cleaved, most likely by signal peptidase. We conclude that the membrane-spanning region of I gamma contains a signal sequence in its amino-terminal half and that hydrophilic residues at the amino-terminal end of a signal sequence can determine cleavage by signal peptidase.

Loss of enzyme activity in a site-directed mutant of influenza neuraminidase compared to expressed wild-type protein

Full-length double-stranded DNA copies of the neuraminidase (NA) gene of influenza virus A/Tokyo/3/67 (N2) and a mutant generated in vitro by site-specific, oligonucleotide-directed mutagenesis with a substitution of leucine for tryptophan at position 178 were cloned into an SV40 late replacement expression vector. Indirect immunofluorescence of cells infected with these recombinant vectors showed the presence of NA protein in the cytoplasm and on the surface of infected cells. Cells expressing the wild-type protein showed neuraminidase enzyme activity for both fetuin, a sialated glycoprotein (mol wt = 50,000) and N-acetylneuraminyl lactose, a trisaccharide (mol wt = 600). This enzyme activity was inhibited by 44% toward N-acetylneuraminyl lactose and by 98% toward fetuin by adding anti-NA antibody before substrate. In contrast, cells expressing the mutant NA had no detectable enzyme activity for either substrate. The conserved nature of the tryptophan at position 178 in all known NA strains, its location in the substrate binding pocket in the three-dimensional structure and the lack of activity of the mutant protein indicate that this residue is essential for enzyme activity.

Segregation of mutant ovalbumins and ovalbumin-globin fusion proteins in Xenopus oocytes. Identification of an ovalbumin signal sequence

The intramolecular signals for chicken ovalbumin secretion were examined by producing mutant proteins in Xenopus oocytes. An ovalbumin complementary DNA clone was manipulated in vitro, and constructs containing altered protein-coding sequences and either the simian virus 40 (SV40) early promoter or Herpes simplex thymidine kinase promoter, were microinjected into Xenopus laevis oocytes. The removal of the eight extreme N-terminal amino acids of ovalbumin had no effect on the segregation of ovalbumin with oocyte membranes nor on its secretion. A protein lacking amino acids 2 to 21 was sequestered in the endoplasmic reticulum but remained strongly associated with the oocyte membranes rather than being secreted. Removal of amino acids 231 to 279, a region previously reported to have membrane-insertion function, resulted in a protein that also entered the endoplasmic reticulum but was not secreted. Hybrid proteins containing at their N terminus amino acids 9 to 41 or 22 to 41 of ovalbumin fused to the complete chimpanzee alpha-globin polypeptide were also sequestered by oocyte membranes. We conclude that the ovalbumin "signal" sequence is internally located within amino acids 22 to 41, and we speculate that amino acids 9 to 21 could be important for the completion of ovalbumin translocation through membranes.