Seth Shaguna, Goodman Andrew L, Compans Richard W
Department of Microbiology and Immunology, Emory University School of Medicine, Atlanta, GA 30322, USA.
J Virol. 2004 Aug;78(16):8513-23. doi: 10.1128/JVI.78.16.8513-8523.2004.
SER virus, a paramyxovirus that is closely related to simian virus 5 (SV5), is unusual in that it fails to induce syncytium formation. The SER virus F protein has an unusually long cytoplasmic tail (CT), and it was previously observed that truncations or specific mutations of this domain result in enhanced syncytium formation. In addition to the long CT, the SER F protein has nine amino acid differences from the F protein of SV5. We previously observed only a partial suppression of fusion in a chimeric SV5 F protein with a CT derived from SER virus, indicating that these other amino acid differences between the SER and SV5 F proteins also play a role in regulating the fusion phenotype. To examine the effects of individual amino acid differences, we mutated the nine SER residues individually to the respective residues of the SV5 F protein. We found that most of the mutants were expressed well and were transported to the cell surface at levels comparable to that of the wild-type SER F protein. Many of the mutants showed enhanced lipid mixing, calcein transfer, and syncytium formation even in the presence of the long SER F protein CT. Some mutants, such as the I310 M, T438S, M489I, T516V, and N529K mutants, also showed fusion at lower temperatures of 32, 25, and 18 degrees C. The residue Asn529 plays a critical role in the suppression of fusion activity, as the mutation of this residue to lysine caused a marked enhancement of fusion. The effect of the N529K mutation on the enhancement of fusion by a previously described mutant, L539,548A, as well as by chimeric SV5/SER F proteins was also dramatic. These results indicate that activation to a fusogenic conformation is dependent on the interplay of residues in the ectodomain, the transmembrane domain, and the CT domain of paramyxovirus F proteins.
SER病毒是一种与猴病毒5（SV5）密切相关的副粘病毒，其不同寻常之处在于它不能诱导合胞体形成。SER病毒F蛋白具有异常长的细胞质尾巴（CT），之前观察到该结构域的截短或特定突变会导致合胞体形成增强。除了长CT外，SER F蛋白与SV5的F蛋白有9个氨基酸差异。我们之前观察到，在具有源自SER病毒CT的嵌合SV5 F蛋白中，融合仅得到部分抑制，这表明SER和SV5 F蛋白之间的这些其他氨基酸差异在调节融合表型中也起作用。为了研究单个氨基酸差异的影响，我们将9个SER残基分别突变为SV5 F蛋白的相应残基。我们发现大多数突变体表达良好，并以与野生型SER F蛋白相当的水平转运到细胞表面。即使存在长的SER F蛋白CT，许多突变体仍显示出增强的脂质混合、钙黄绿素转移和合胞体形成。一些突变体，如I310M、T438S、M489I、T516V和N529K突变体，在32、25和18摄氏度的较低温度下也显示出融合。Asn529残基在融合活性的抑制中起关键作用，因为该残基突变为赖氨酸会导致融合显著增强。N529K突变对先前描述的突变体L539,548A以及嵌合SV5/SER F蛋白增强融合的作用也很显著。这些结果表明，向融合构象的激活取决于副粘病毒F蛋白的胞外结构域、跨膜结构域和CT结构域中残基的相互作用。
