Table 1.
Peptide counts in VP7 interactome.This table summarizes the peptide counts of proteins identified in VP7 interactome. The target genes, their corresponding symbols, and the number of peptides detected for each are shown.
Fig 1.
SPC interacts with rotavirus VP7.
(A) Interactome of VP7 and host proteins. The proteins connected by dotted lines belong to the same family based on published interactome studies. The width of the arrows indicates the strength of the interaction between host proteins and DS1-VP7. (B) Co-localization analysis of VP7 and REEP5, SEC11C, SPCS2, VP3 and PFDN4, NSP1 and SAMD9, VP6 and ECE1. Results are shown as overlap coefficients based on images in S1 Fig. (C) Co-IP of SPCS1 with VP7. As SPC was a complex, SPCS1 was representative of the complex in the co-ip assay. HEK293 cells were co-transfected with plasmids expressing FLAG-tagged SPCS1 and GFP-tagged DS1-VP7. Cell lysates were immunoprecipitated with an anti-GFP antibody and corresponding isotype control. The resulting precipitates and whole-cell lysates used for immunoprecipitation were examined by immunoblot with anti-GFP and anti-FLAG antibodies. (D) Co-IP of endogenous SPCS1 with VP7. HEK293 cells were transfected with plasmids expressing GFP-tagged EV or DS1-VP7. Cell lysates were immunoprecipitated with an anti-GFP antibody or isotype IgG control. The resulting precipitates and whole-cell lysates used for immunoprecipitation were examined by immunoblot with anti-SPCS1 and anti-GFP antibodies.
Fig 2.
Loss of SPC impairs rotavirus growth.
(A) WT, SPCS1 KO HEK293T, SPCS1 KO cells transfected with empty vector or FLAG-tagged SPCS1 were infected with RRV at an MOI of 3, and cells were collected for western blot analysis at 12 hpi using anti-FLAG and SPCS1 antibodies to confirm the protein level of SPCS1. (B) The lysates and supernatants were collected for FFU assays to determine the titers. (C) WT and SPCS1 KO HEK293T cells were infected with RRV, SA11 and UK strains at an MOI of 3. At 12 hpi, all the cell lysates and supernatants were collected for FFU assay to detect the titers. (D) WT and SPCS1 KO Huh7.5 cells were infected with RRV, SA11 and UK strains at an MOI of 3, respectively. At 12 hpi, all the cell lysates and supernatants were collected for FFU assay to detect the titers. (E) WT and SPCS1 KO HEK293T cells were transfected with siRNAs targeted against SPCS2, SEC11A, SEC11C and a scrambled siRNA at the concentration of 20 nM. At 48 hpi, cells were infected with RRV at an MOI of 3. The cell lysates and supernatants were collected for FFU assay to detect the titers at 12 hpi. The results are the averages of data in three independent experiments and plotted as mean ± SD. Statistical significance was determined using two-way ANOVA with Sidak’s multiple comparisons test (**, P < 0.01; ***, P < 0.001, **** P < 0.0001).
Fig 3.
SPCS1 does not regulate RNA and protein level during rotavirus infection.
(A) WT and SPCS1 KO HEK293T cells were infected with RRV at an MOI of 3. At 4, 8, 12 hpi, the cells were collected for qRT-PCR analysis of viral mRNA level by detecting NSP5. The NSP5 level was normalized to that of GAPDH. Results are the average of data in triplicates and plotted as mean ± SD. (B) WT and SPCS1 KO HEK293T cells were infected with RRV at an MOI of 3. At 4, 8, 12 hpi, the infected cells and mock cells were harvested for western blot analysis of viral protein level by detecting VP6 and VP7 protein. (C) WT and SPCS1 KO HEK293T cells were infected with RRV at an MOI of 3. At 12 hpi, an immunofluorescence assay was performed to detect the viroplasms in the infected cells. Cells were fixed and probed with in-house polyclonal antibodies against NSP2 and NSP5; the positive cells were shown as green fluorescence. Nuclei were counterstained with DAPI. Representative data from one of three independent experiments are shown. Scale bar, 1 μm. (D) The numbers of viroplasms in RV-infected WT and SPCS1 KO HEK293T cells were quantified using images from (C). (E) The sizes of viroplasms in RV-infected WT and SPCS1 KO HEK293T cells were quantified using images from (C) by Image J. The results are the averages of data in three different cells and plotted as mean ± SD. Statistical significance was determined using two-way ANOVA with Sidak’s multiple comparisons test.
Fig 4.
Loss of SPCS1 hinders rotavirus TLP formation.
(A) WT and SPCS1 KO HEK293T cells were infected with RRV at an MOI of 3 for 16 hours, and the supernatants were subjected to sucrose cushion followed by CsCl gradient ultracentrifugation. (B) Western blot validation of sucrose cushion products by detecting DLPs and TLPs using guinea pig polyclonal serum against TLP. (C) Electron micrograph (low magnification) of RRV-infected HEK293T WT cells. ‘V’ indicates the viroplasm. (D) Enlarged view of the area boxed in red in panel (C). Blue arrows indicate TLPs. (E) Electron micrograph (low magnification) of RRV-infected SPCS1 KO cells. ‘V’ indicates the viroplasm. (F) Enlarged view of the area boxed in red in panel (E). Magenta arrows indicate abnormal morphology of TLPs. Scale bars: low magnification, 50 nm; high magnification, 100 nm.
Fig 5.
SPC is important for rotavirus VP7 maturation.
(A) Schematic diagram of rotavirus VP7, cleavage site was indicated by an arrow. (B) WT and SPCS1 KO HEK293T cells were transfected with plasmids expressing -EV, -WT VP7, -A50V mutant VP7, -non-structural proteins NSP4, respectively. Those plasmids were all tagged with GFP. At 24 hours post-transfection, cells were harvested for western blot analysis by probing GFP. (C) Quantification of the cleavage ratio of WT and A50V mutant VP7 proteins in WT and SPCS1 KO HEK293T cells. The results are the averages of data in three independent experiments and plotted as mean ± SD. Statistical significance was determined by two-way ANOVA with Sidak’s multiple comparisons test (***, P < 0.001, **** P < 0.0001).
Fig 6.
Residue E256 mediates VP7 binding to SPC.
(A) Alpha-Fold 3 prediction of VP7 with SPC complex (SPCS1, SPCS2, SPCS3, and SEC11A). (B) The magnification of the interaction site of VP7 and SPCS3 from (A). (C) Co-IP of SPCS1 with VP7 mutants. HEK293 cells were co-transfected with plasmids expressing SPCS1-FLAG with GFP-WT VP7, -E256R mutant VP7, -NSP4 and -EV, respectively. Cell lysates of transfected cells were immunoprecipitated with anti-GFP antibody or its corresponding isotype IgG control. The resulting precipitates and whole-cell lysates used for immunoprecipitation were examined by immunoblot using anti-GFP and anti-FLAG antibodies. (D) WT and SPCS1 KO HEK293T cells were infected with rSA11-WT or rSA11-E256R at an MOI of 3. At 12 hpi, all the cell lysates and supernatants were collected for FFU assay to detect the titers. Statistical significance was determined by two-way ANOVA with Sidak’s multiple comparisons test (***, P ＜0.001, **** P ＜0.0001). (E) Samples were collected from WT HEK293T cells infected with rSA11-VP7-WT and rSA11-VP7-E256R in three independent experiments. The purified PCR products targeting the VP7 gene were subjected to Sanger sequencing, and the results are shown in the chart (right panel). Direct sequencing of VP7 from rSA11-VP7-WT and rSA11-VP7-E256R viruses served as controls. The red box indicates the codon of mutated amino acids.
Table 2.
Primer sequences used in this study. This table lists the sequences of primers and probe used for qPCR analysis in the study. Primer names are listed along with their corresponding sequences in the 5’ to 3’ direction. The NSP5 probe is labeled with a fluorescent dye FAM and quencher IABRQSP for qPCR detection using a TaqMan probe, while the remaining primers targeting GAPDH, SEC11A, SEC11C, SPCS2, are used for SYBR Green qPCR detection.