# Signal peptidase complex mediates rotavirus VP7 processing and virion assembly

## TL;DR

This study shows that the signal peptidase complex (SPC) is essential for processing the rotavirus protein VP7, which is crucial for virus assembly and infectivity.

## Contribution

The study identifies the SPC as a novel host factor required for rotavirus VP7 maturation and assembly, revealing a new antiviral target.

## Key findings

- SPC is essential for cleaving the VP7 signal peptide, and its loss leads to impaired virion assembly.
- Mutation of VP7 residue E256 disrupts SPC binding and reduces viral infectivity.
- SPC is a potential broad-spectrum antiviral target for viruses like rotavirus, flavivirus, and HTLV-1.

## Abstract

For viruses that replicate in the proximity of or bud at the endoplasmic reticulum (ER) associated membranes, proper processing of their glycoproteins is critical for successful infection. Rotavirus outer capsid protein VP7 is an ER-resident protein. However, its N-terminal signal peptide is removed by an unknown proteolytic mechanism. In this study, we leveraged tandem affinity purification followed by high-resolution mass spectrometry to profile host proteins that interact with VP7. We identified members of the signal peptidase complex (SPC) family as important host factors that facilitate rotavirus infection. CRISPR knockout or siRNA knockdown of distinct SPC subunits resulted in significant decrease in infectious rotavirus titers in a viral strain- and cell type-independent manner. While viral transcription, translation, and replication were not altered in the absence of SPC, we observed formation of abnormal viral particles by transmission electron microscopy (TEM) in SPCS1 knockout cells. Mechanistically, loss of SPC proteins led to inefficient cleavage of VP7 signal peptide and severely impaired the final steps of virion maturation and assembly. Additionally, we identified residue E256 within VP7 as a key site for SPC binding. An E to R mutation abolished VP7 interaction with SPC and subsequently led to reduced viral infectivity. Taken together, these findings define SPC as a novel regulator of VP7 maturation and rotavirus assembly and highlight its role as a novel cellular target for potentially broad-spectrum antiviral therapeutic development.

Previous reports demonstrated that SPC is involved in the replication cycles of several members of the flavivirus family and human T-cell leukemia virus type 1 (HTLV-1). As an ER-resident outer capsid protein, rotavirus VP7 must undergo proper post-translational modifications, including the cleavage of its N-terminal signal peptide, to be functionally incorporated into mature virions. However, the processing mechanism remains unknown. Our study identifies a novel role for SPC as an essential regulator of rotavirus assembly by mediating the cleavage of the VP7 signal peptide. Loss of SPC impairs VP7 signal peptide cleavage and maturation, thereby disrupting correct virion assembly and reducing infectious particle production. Using Alphafold3, we predicted the VP7 residue E256 to be at the binding interface with SPC complex. Experimentally, mutation of glutamic acid to arginine (E256R) substantially weakens this interaction and results in reduced viral propagation. Our findings unveil a novel post-translational checkpoint in rotavirus regulated by SPC and underscore the promise of SPC as a broad-spectrum antiviral target, especially for rotavirus, flavivirus, and HTLV-1, whose viral glycoproteins and structural proteins require SPC processing for proper maturation.