Simian virus 5 membrane protein maturation: expression in virus-infected cells and from a eukaryotic vector

Parainfluenza virus 5 m protein interaction with host protein 14-3-3 negatively affects virus particle formation

Paramyxovirus matrix (M) proteins organize virus assembly, linking viral glycoproteins and viral ribonucleoproteins together at virus assembly sites on cellular membranes. Using a yeast two-hybrid screening approach, we identified 14-3-3 as a binding partner for the M protein of parainfluenza virus 5 (PIV5). Binding in both transfected and PIV5-infected cells was confirmed by coimmunoprecipitation and was mapped to a C-terminal region within the M protein, namely, 366-KTKSLP-371. This sequence resembles known 14-3-3 binding sites, in which the key residue for binding is a phosphorylated serine residue. Mutation of S369 within the PIV5 M protein disrupted 14-3-3 binding and improved the budding of both virus-like particles (VLPs) and recombinant viruses, suggesting that 14-3-3 binding impairs virus budding. 14-3-3 protein overexpression reduced the budding of VLPs. Using (33)P labeling, phosphorylated M protein was detected in PIV5-infected cells, and this phosphorylation was nearly absent in cells infected with a recombinant virus harboring an S369A mutation within the M protein. Assembly of the M protein into clusters and filaments at infected cell surfaces was enhanced in cells infected with a recombinant virus defective in 14-3-3 binding. These findings support a model in which a portion of M protein within PIV5-infected cells is phosphorylated at residue S369, binds the 14-3-3 protein, and is held away from sites of virus budding.

Alteration of Sendai virus morphogenesis and nucleocapsid incorporation due to mutation of cysteine residues of the matrix protein

The matrix (M) protein of Sendai virus (SeV) has five cysteine residues, at positions 83, 106, 158, 251, and 295. To determine the roles of the cysteine residues in viral assembly, we generated mutant M cDNA possessing a substitution to serine at one of the cysteine residues or at all of the cysteine residues. Some mutant M proteins were unstable when expressed in cultured cells, suggesting that cysteine residues affect protein stability, probably by disrupting the proper conformation. In an attempt to generate virus from cDNA, SeV M-C(83)S, SeV M-C(106)S, and SeV M-C(295)S were successfully recovered from cDNA, while recombinant SeVs possessing other mutations were not. SeV M-C(83)S and SeV M-C(106)S had smaller virus particles than did the wild-type SeV, whereas SeV M-C(295)S had larger and heterogeneously sized particles. Furthermore, SeV M-C(106)S had a significant amount of empty particles lacking nucleocapsids. These results indicate that a single-point mutation at a cysteine residue of the M protein affects virus morphology and nucleocapsid incorporation, showing direct involvement of the M protein in SeV assembly. Cysteine-dependent conformation of the M protein was not due to disulfide bond formation, since the cysteines were shown to be free throughout the viral life cycle.

Proper spacing between heptad repeat B and the transmembrane domain boundary of the paramyxovirus SV5 F protein is critical for biological activity

The paramyxovirus, simian virus 5, fusion (F) protein contains seven amino acids between heptad repeat B (a domain required for a biologically active fusion protein) and the presumptive boundary of the transmembrane (TM) domain. The role of the seven membrane proximal residues in stability and fusion promotion was examined by construction of a series of insertion, substitution, and deletion mutants, as manipulation of this region to enable proteolytic cleavage would facilitate production of a soluble F protein. The majority of the mutant F proteins both oligomerized and had kinetics of intracellular transport similar to those of wild-type (wt) F protein. All mutant F proteins were expressed at the cell surface at or near the same level as the wt F protein. However, by using both a qualitative lipid mixing assay and a quantitative content mixing assay for membrane fusion, it was found that mutant F proteins containing insertions in the region between heptad repeat B and the TM domain were unable to induce fusion, whereas the mutant F proteins containing substitutions in this region, together with three of the four mutants with deletions in this region, could induce fusion. Four of the F protein mutants contained a Factor Xa cleavage site, IEGR; however, Factor Xa treatment of cell surfaces released either none or only very small amounts (< 1% of total protein) of the soluble heterodimer F1 + F2. As an alternative method of generating soluble F protein, a glycosyl phosphatidylinositol (GPI) anchor was added to the F protein at three membrane-proximal positions. The highest level of surface expression was observed when the final molecule did not contain a significant insertion of amino acids into the membrane proximal region. Two F-GPI mutants reached the surface at approximately 20% of the levels seen with the wt F protein, and approximately 25% of the cell surface population of these mutants could be cleaved with phosphatidylinositol phospholipase C (PI-PLC) to yield soluble F protein. However, all the F-GPI mutants oligomerized aberrantly and failed to promote fusion. Taken together, these data indicate that the spacing of the region immediately adjacent to the presumptive boundary of the TM domain is extremely important for the fusogenic activity of the SV5 F protein.

Phosphorylation of the Sendai virus M protein is not essential for virus replication either in vitro or in vivo

A large proportion of intracellular Sendai virus (SeV) M proteins is phosphorylated, but in mature virions the M protein is not phosphorylated or dephosphorylated. Phosphorylated M protein in cells is bound to the cytoskeletal components more firmly than unphosphorylated M protein. Thus it has been hypothesized that M protein phosphorylation plays an important role in the virus life cycle, especially in the step of maturation. Here, a transient expression-mutation experiment of the M gene demonstrated that a change of the Ser residue at the 70th position from the N-terminus to Ala (S70A) totally abolished M protein phosphorylation, strongly suggesting that this residue is phosphorylated. The mutated M gene was then placed in the corresponding region in the cDNA plasmid which generates a full-length antigenome SeV RNA, and a mutant SeV M-S70A was successfully recovered from the cDNA. This mutant virus was indeed defective in M protein phosphorylation but did not differ at all from the wild-type SeV recovered from the parental cDNA either in the replication kinetics and plaque morphology in cultured cells or in in vivo replication and pathogenicity for mice. We thus concluded that no phosphorylation of the M protein was required for SeV replication either in vitro or in vivo.

The role of the cytoplasmic tail region of influenza virus hemagglutinin in formation and growth of fusion pores

The effect of the cytoplasmic tail of influenza hemagglutinin (HA) (H3 subtype) on fusion kinetics and pore growth was examined An SV40 recombinant virus was used to express wild-type (WT) HA and HA mutants containing changes in the HA cytoplasmic tail. HA and its mutants were expressed in CV-1 cells and the ability of these cells to fuse to either red blood cells (RBCs) or planar bilayer membranes was determined quantitatively. The percentage of cells expressing HA and the levels of expression were the same for WT HA or HA lacking its cytoplasmic tail (CT-), and for a mutant, MAY, in which the three HA C-terminal cysteine residues were replaced to block the addition of palmitate. When RBCs were colabeled with large and small aqueous dyes and fused to CV-1 cells expressing WT HA, transfer of the large dye was significantly slower and extent of transfer was lower than that of the small dye, indicating that pores did not expand quickly to large diameters. An absence of the HA cytoplasmic tail did not alter the time course of spread for either dye. When CV-1 cells expressing WT HA were fused to planar membranes, small pores tended to open and close repetitively ("flicker") before a pore would continue to either grow irreversibly to large conductances or grow to intermediate sizes and then contract. For HA mutants CT- and MAY, flickering was less likely to occur, but these pores did evolve in a manner identical to WT HA postflicker pores. We conclude that palmitate covalently linked to cysteine residues of the HA cytoplasmic tail is required for pore flickering, but that the tail does not play an important role in subsequent pore enlargement.

Association of influenza virus NP and M1 proteins with cellular cytoskeletal elements in influenza virus-infected cells

We have investigated the association of the influenza virus matrix (M1) and nucleoprotein (NP) with the host cell cytoskeletal elements in influenza virus-infected MDCK and MDBK cells. At 6.5 h postinfection, the newly synthesized M1 was Triton X-100 (TX-100) extractable but became resistant to TX-100 extraction during the chase with a t1/2 of 20 min. NP, on the other hand, acquired TX-100 resistance immediately after synthesis. Significant fractions of both M1 and NP remained resistant to differential detergent (Triton X-114, 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate [CHAPS], octylglucoside) extraction, suggesting that M1 and NP were interacting with the cytoskeletal elements. However, the high-molecular-weight form of the viral transmembrane protein hemagglutinin (HA), which had undergone complex glycosylation, also became resistant to TX-100 extraction but was sensitive to octylglucoside detergent extraction, indicating that HA, unlike M1 or NP, was interacting with TX-100-insoluble lipids and not with cytoskeletal elements. Morphological analysis with cytoskeletal disrupting agents demonstrated that M1 and NP were associated with microfilaments in virus-infected cells. However, M1, expressed alone in MDCK or HeLa cells from cloned cDNA or coexpressed with NP, did not become resistant to TX-100 extraction even after a long chase. NP, on the other hand, became TX-100 insoluble as in the virus-infected cells. M1 also did not acquire TX-100 insolubility in ts 56 (a temperature-sensitive mutant with a defect in NP protein)-infected cells at the nonpermissive temperature. Furthermore, early in the infectious cycle in WSN-infected cells, M1 acquired TX-100 resistance very slowly after a long chase and did not acquire TX-100 resistance at all when chased in the presence of cycloheximide. On the other hand, late in the infectious cycle, M1 acquired TX-100 resistance when chased in either the presence or absence of cycloheximide. Taken together, these results demonstrate that M1 and NP interact with host microfilaments in virus-infected cells and that M1 requires other viral proteins or subviral components (possibly viral ribonucleoprotein) for interaction with host cytoskeletal components. The implication of these results for viral morphogenesis is discussed.

Sendai virus M protein binds independently to either the F or the HN glycoprotein in vivo

We have analyzed the mechanism by which M protein interacts with components of the viral envelope during Sendai virus assembly. Using recombinant vaccinia viruses to selectively express combinations of Sendai virus F, HN, and M proteins, we have successfully reconstituted M protein-glycoprotein interaction in vivo and determined the molecular interactions which are necessary and sufficient to promote M protein-membrane binding. Our results showed that M protein accumulates on cellular membranes via a direct interaction with both F and HN proteins. Specifically, our data demonstrated that a small fraction (8 to 16%) of M protein becomes membrane associated in the absence of Sendai virus glycoproteins, while > 75% becomes membrane bound in the presence of both F and HN proteins. Selective expression of M protein together with either F or HN protein showed that each viral glycoprotein is individually sufficient to promote efficient (56 to 73%) M protein-membrane binding. Finally, we observed that M protein associates with cellular membranes in a time-dependent manner, implying a need for either maturation or transport before binding to glycoproteins.

Biological activity of paramyxovirus fusion proteins: factors influencing formation of syncytia

The fusion (F) and hemagglutinin-neuraminidase (HN) glycoproteins of the paramyxovirus simian virus 5 (SV5) were expressed individually or coexpressed in CV-1 cells by using SV40-based vectors and recombinant vaccinia viruses. The extent of detectable fusion in a syncytium formation assay was found to be affected by the expression system used. In addition, when HN was coexpressed with F, it was found that the expression vector system influenced the contribution of HN in forming syncytia. The abilities of the SV5, human parainfluenza virus type 3, and Newcastle disease virus F glycoproteins to cause fusion, when expressed alone or coexpressed with HN, were directly compared by using the SV40-based vector system in CV-1 cells. The F proteins exhibited various degrees of fusion activity independent of HN expression, but the formation of syncytia could be enhanced to different extents by the coexpression of the homotypic HN protein.

Sendai virus M protein is found in two distinct isoforms defined by monoclonal antibodies

The use of a monoclonal antibody defines a subset of Sendai virus M protein representing about 30% of total. This M protein acquires, during the hour following synthesis, an epitope not present on the bulk of M. This epitope maturation is observed in acutely as well as in persistently infected cells. It takes place in vivo in absence of other viral proteins, but it is not observed when the protein is synthesized in a reticulocyte lysate. Epitope maturation does not appear to result from phosphorylation, acylation or disulfide bond formation. If immunofluorescent staining seems to indicate a preferential association of this subset of M protein with nucleocapsids, this is not confirmed by immunogold staining or by nucleocapsid isolation. Incubation of cytoplasmic extracts or of purified M protein in conditions which do not favor M to M protein association results in a relative increase of M protein carrying the maturing epitope. It is concluded that M protein exists in two distinct isoforms.

Studies on the fusion peptide of a paramyxovirus fusion glycoprotein: roles of conserved residues in cell fusion

The role of residues in the conserved hydrophobic N-terminal fusion peptide of the paramyxovirus fusion (F) protein in causing cell-cell fusion was examined. Mutations were introduced into the cDNA encoding the simian virus 5 (SV5) F protein, the altered F proteins were expressed by using an eukaryotic vector, and their ability to mediate syncytium formation was determined. The mutant F proteins contained both single- and multiple-amino-acid substitutions, and they exhibited a variety of intracellular transport properties and fusion phenotypes. The data indicate that many substitutions in the conserved amino acids of the simian virus 5 F fusion peptide can be tolerated without loss of biological activity. Mutant F proteins which were not transported to the cell surface did not cause cell-cell fusion, but all of the mutants which were transported to the cell surface were fusion competent, exhibiting fusion properties similar to or better than those of the wild-type F protein. Mutant F proteins containing glycine-to-alanine substitutions had altered intracellular transport characteristics, yet they exhibited a great increase in fusion activity. The potential structural implications of this substitution and the possible importance of these glycine residues in maintaining appropriate levels of fusion activity are discussed.

Analysis in vivo of GRP78-BiP/substrate interactions and their role in induction of the GRP78-BiP gene

The endoplasmic reticulum (ER)-localized chaperone protein, GRP78-BiP, is involved in the folding and oligomerization of secreted and membrane proteins, including the simian virus 5 hemagglutinin-neuraminidase (HN) glycoprotein. To understand this interaction better, we have constructed a series of HN mutants in which specific portions of the extracytoplasmic domain have been deleted. Analysis of these mutant polypeptides expressed in CV-1 cells have indicated that GRP78-BiP binds to selective sequences in HN and that there exists more than a single site of interaction. Mutant polypeptides have been characterized that are competent and incompetent for association with GRP78-BiP. These mutants have been used to show that the induction of GRP78-BiP synthesis due to the presence of nonnative protein molecules in the ER is dependent on GRP78-BiP complex formation with its substrates. These studies have implications for the function of the GRP78-BiP protein and the mechanism by which the gene is regulated.