A mutant fusion (F) protein of simian virus 5 induces hemagglutinin-neuraminidase-independent syncytium formation despite the internalization of the F protein

The Hemagglutinin-Neuraminidase (HN) Head Domain and the Fusion (F) Protein Stalk Domain of the Parainfluenza Viruses Affect the Specificity of the HN-F Interaction

Membrane fusion by the parainfluenza viruses is induced by virus-specific functional interaction between the attachment protein (HN) and the fusion (F) protein. This interaction is thought to be mediated by transient contacts between particular amino acids in the HN stalk domain and those in the F head domain. However, we recently reported that replacement of specified amino acids at or around the dimer interface of the HN head domain remarkably affected the F protein specificity. We then intended to further investigate this issue in the present study and revealed that the HPIV2 HN protein can be converted to an SV41 HN-like protein by substituting at least nine amino acids in the HPIV2 HN head domain with the SV41 HN counterparts in addition to the replacement of the stalk domain, indicating that specified amino acids in the HN head domain play very important roles in determining the specificity of the HN-F interaction. On the other hand, we previously reported that the PIV5 F protein can be converted to an SV41 F-like protein by replacing 21 amino acids in the head domain of the PIV5 F protein with those of the SV41 F protein. We then intended to further investigate this issue in the present study and found that replacement of 15 amino acids in the stalk domain in addition to the replacement of the 21 amino acids in the head domain of the PIV5 F protein resulted in creation of a more SV41 F-like protein, indicating that specified amino acids in the F stalk domain play important roles in determining the specificity of the HN-F interaction. These results suggest that the conformations of the HN stalk domain and the F head domain are dependent on the structures of the HN head domain and the F stalk domain, respectively. Presumably, the conformations of the former domains, which are considered directly involved in the HN-F interaction, can be modified by subtle changes in the structure of the latter domains, resulting in an altered specificity for the interacting partners.

Full conversion of the hemagglutinin-neuraminidase specificity of the parainfluenza virus 5 fusion protein by replacement of 21 amino acids in its head region with those of the simian virus 41 fusion protein

For most parainfluenza viruses, a virus type-specific interaction between the hemagglutinin-neuraminidase (HN) and fusion (F) proteins is a prerequisite for mediating virus-cell fusion and cell-cell fusion. The molecular basis of this functional interaction is still obscure partly because it is unknown which region of the F protein is responsible for the physical interaction with the HN protein. Our previous cell-cell fusion assay using the chimeric F proteins of parainfluenza virus 5 (PIV5) and simian virus 41 (SV41) indicated that replacement of two domains in the head region of the PIV5 F protein with the SV41 F counterparts bestowed on the PIV5 F protein the ability to induce cell-cell fusion on coexpression with the SV41 HN protein while retaining its ability to induce fusion with the PIV5 HN protein. In the study presented here, we furthered the chimeric analysis of the F proteins of PIV5 and SV41, finding that the PIV5 F protein could be converted to an SV41 HN-specific chimeric F protein by replacing five domains in the head region with the SV41 F counterparts. The five SV41 F-protein-derived domains of this chimera were then divided into 16 segments; 9 out of 16 proved to be not involved in determining its specificity for the SV41 HN protein. Finally, mutational analyses of a chimeric F protein, which harbored seven SV41 F-protein-derived segments, revealed that replacement of at most 21 amino acids of the PIV5 F protein with the SV41 F-protein counterparts was enough to convert its HN protein specificity.

Identification of domains on the fusion (F) protein trimer that influence the hemagglutinin-neuraminidase specificity of the f protein in mediating cell-cell fusion

For most paramyxoviruses, virus type-specific interaction between fusion (F) protein and attachment protein (hemagglutinin-neuraminidase [HN], hemagglutinin [H], or glycoprotein [G]) is a prerequisite for mediating virus-cell fusion and cell-cell fusion. Our previous cell-cell fusion assay using the chimeric F proteins of human parainfluenza virus 2 (HPIV2) and simian virus 41 (SV41) suggested that the middle region of the HPIV2 F protein contains the site(s) that determines its specificity for the HPIV2 HN protein. In the present study, we further investigated the sites of the F protein that could be critical for determining the HN protein specificity. By analyzing the reported structure of the F protein of parainfluenza virus 5 (PIV5), we found that four major domains (M1, M2, M3, and M4) and five minor domains (A to E) in the middle region of the PIV5 F protein were exposed on the trimer surface. We then replaced these domains with the SV41 F counterparts individually or in combination and examined whether the resulting chimeras could mediate cell-cell fusion when coexpressed with the SV41 HN protein. The results showed that a chimera designated M(1+2), which harbored SV41 F-derived domains M1 and M2, mediated cell-cell fusion with the coexpressed SV41 HN protein, suggesting that these domains are involved in determining the HN protein specificity. Intriguingly, another chimera which harbored the SV41 F-derived domain B in addition to domains M1 and M2 showed increased specificity for the SV41 HN protein compared to that of M(1+2), although it was capable of mediating cell-cell fusion by itself.

Effects of multiple amino acids of the parainfluenza virus 5 fusion protein on its haemagglutinin-neuraminidase-independent fusion activity

The fusion (F) protein of parainfluenza virus 5 (PIV-5) strain W3A is able to induce cell fusion when it is expressed alone in baby hamster kidney cells, whilst the F protein of PIV-5 strain WR induces cell fusion only when co-expressed with the haemagglutinin-neuraminidase (HN) protein. It has been shown previously that when Leu-22 of the WR F protein is replaced with the W3A F counterpart (Pro-22), the resulting mutant L22P exhibits HN-independent fusion activity. Furthermore, previous chimeric analysis between L22P and the F protein of PIV-5 strain T1 has suggested that Glu-132 also contributes to the HN-independent fusion activity of L22P. It was shown here that substitution of Glu-132 of L22P with various amino acids including the T1 F protein counterpart (Lys-132) resulted in a reduction in fusion activity, whereas substitution with Asp was the exception in being tolerated. Interestingly, reduced fusion activity of an L22P mutant that harboured the E132K substitution could be restored by an additional D416K substitution but not by a D416E mutation, suggesting that the presence of the same charge at positions 132 and 416 is important for the HN-independent fusion activity. In contrast, substitution of Leu-22 of the WR F protein with various amino acids except those with aliphatic side chains resulted in acquisition of fusion activity, suggesting that the HN dependence of the WR F protein in the induction of cell fusion is attributable to the hydrophobicity of Leu-22. These results indicate that at least three amino acids are involved in the HN-independent fusion activity of the PIV-5 F protein.

Effects of hemagglutinin-neuraminidase protein mutations on cell-cell fusion mediated by human parainfluenza type 2 virus

The monoclonal antibody M1-1A, specific for the hemagglutinin-neuraminidase (HN) protein of human parainfluenza type 2 virus (HPIV2), blocks virus-induced cell-cell fusion without affecting the hemagglutinating and neuraminidase activities. F13 is a neutralization escape variant selected with M1-1A and contains amino acid mutations N83Y and M186I in the HN protein, with no mutation in the fusion protein. Intriguingly, F13 exhibits reduced ability to induce cell-cell fusion despite its multistep replication. To investigate the potential role of HPIV2 HN protein in the regulation of cell-cell fusion, we introduced these mutations individually or in combination to the HN protein in the context of recombinant HPIV2. Following infection at a low multiplicity, Vero cells infected with the mutant virus H-83/186, which carried both the N83Y and M186I mutations, remained as nonfused single cells at least for 24 h, whereas most of the cells infected with wild-type virus mediated prominent cell-cell fusion within 24 h. On the other hand, the cells infected with the mutant virus, carrying either the H-83 or H-186 mutation, mediated cell-cell fusion but less efficiently than those infected with wild-type virus. Irrespective of the ability to cause cell-cell fusion, however, every virus could infect all the cells in the culture within 48 h after the initial infection. These results indicated that both the N83Y and M186I mutations in the HN protein are involved in the regulation of cell-cell fusion. Notably, the limited cell-cell fusion by H-83/186 virus was greatly promoted by lysophosphatidic acid, a stimulator of the Ras and Rho family GTPases.