Delayed activation of altered fusion glycoprotein in a chronic measles virus variant that causes subacute sclerosing panencephalitis
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1Department of Microbiology, University of Washington School of Medicine, Seattle 98195, USA.
Abstract:
We compared the intracellular processing of the fusion (F) glycoproteins of an acute measles virus (MV) Nagahata strain and its relative Biken strain that caused subacute sclerosing panencephalitis (SSPE), Nagahata strain synthesizes a precursor F0 which acquires three asparagine (N)-linked oligosaccharide chains sequentially in 1 h. One oligosaccharide chain on the partially glycosylated F0 is less accessible to endo-beta-N-acetylglucosaminidase H (endo-H) but becomes accessible as the protein becomes fully glycosylated, suggesting a protein conformational change. Biken strain SSPE virus synthesizes a similarly glycosylated F0. However, one oligosaccharide chain on the Biken F0 remains less accessible to endo-H even after the protein is fully glycosylated. The Nagahata F0 is cleaved into the F1 and F2 subunits with a half life of 1 h. The Biken F0 is cleaved with a half life of 4 h. We cloned the F genes of Nagahata and Biken strains and showed by transfection that the defect causing delayed cleavage of F0 resides in the Biken F gene. Sequence analysis predicts a mutation in the cleavage recognition sequence, a truncated carboxyl-terminus, and multiple mutations in F1 of the Biken F protein. Expression of chimeric F genes showed the mutated cleavage recognition sequence and the carboxyl-terminal truncation do not delay cleavage of F0. Instead, delayed F0 cleavage is due to multiple mutations in the extracellular domain of F1, and four amino acid substitutions near the transmembrane region impair endo-H access to the oligosaccharide chain. These results provide detailed information on the normal maturation process of the F protein of MV and additional clues to the mechanisms of MV persistence in the CNS.
Insights
We compared measles virus (MV) strains, finding that the Biken strain’s fusion (F) protein has delayed processing and cleavage due to mutations, potentially explaining its persistence in the central nervous system (CNS).
Area of Science:
- Virology
- Molecular Biology
- Neuroscience
Background:
- Measles virus (MV) infection can lead to persistent central nervous system (CNS) disease, such as subacute sclerosing panencephalitis (SSPE).
- The MV fusion (F) glycoprotein is critical for viral entry and cell-to-cell spread, and its proper processing is essential for viral replication.
- Differences in F protein processing between acute and persistent MV strains may contribute to viral pathogenesis.
Purpose of the Study:
- To compare the intracellular processing of the fusion (F) glycoproteins between an acute MV strain (Nagahata) and a strain causing SSPE (Biken).
- To identify the molecular basis for differences in F protein maturation and cleavage between these MV strains.
- To elucidate mechanisms underlying MV persistence in the CNS.
Main Methods:
- Intracellular processing and glycosylation of MV F glycoproteins were analyzed using techniques like endo-beta-N-acetylglucosaminidase H (endo-H) susceptibility assays.
- F genes from Nagahata and Biken strains were cloned and transfected into cells to assess their function.
- Chimeric F genes were constructed and expressed to pinpoint specific mutations responsible for altered F protein processing.
Main Results:
- The Biken strain F0 precursor acquired N-linked oligosaccharide chains similarly to the Nagahata strain, but one chain remained less accessible to endo-H even after full glycosylation.
- Cleavage of Biken strain F0 into F1 and F2 subunits was significantly delayed (half-life of 4 h) compared to the Nagahata strain (half-life of 1 h).
- Mutations within the extracellular domain of F1, specifically four amino acid substitutions near the transmembrane region, were identified as the cause of delayed F0 cleavage and impaired endo-H accessibility in the Biken strain.
Conclusions:
- Specific mutations in the F1 extracellular domain of the Biken MV strain lead to delayed F0 cleavage and altered glycosylation.
- These processing defects in the Biken strain F protein may contribute to MV persistence in the CNS.
- Understanding these molecular differences provides insights into the pathogenesis of SSPE and MV CNS infections.