The ISG15 conjugation system broadly targets newly synthesized proteins: implications for the antiviral function of ISG15

Abstract

ISG15 is an interferon-induced and antiviral ubiquitin-like protein (Ubl). Herc5, the major E3 enzyme for ISG15, mediates the ISGylation of more than 300 proteins in interferon-stimulated cells. In addressing this broad substrate selectivity of Herc5, we found that: (1) the range of substrates extends even further and includes many exogenously expressed foreign proteins, (2) ISG15 conjugation is restricted to newly synthesized pools of proteins, and (3) Herc5 is physically associated with polyribosomes. These results lead to a model for ISGylation in which Herc5 broadly modifies newly synthesized proteins in a cotranslational manner. This further suggests that, in the context of an interferon-stimulated cell, newly translated viral proteins may be primary targets of ISG15. Consistent with this, we demonstrate that ISGylation of human papillomavirus (HPV) L1 capsid protein has a dominant-inhibitory effect on the infectivity of HPV16 pseudoviruses.

Graphical abstract

Figure 1

Detection of ISG15 Conjugates in IFN-β-Treated HeLa Cells (A) Total ISG15 conjugates in HeLa cells. Cells were either untreated or treated with IFN-β for 48 hr. Cell extracts were analyzed by SDS-PAGE and immunoblotting with anti-ISG15 antibody. (B) ISG15 modification of constitutively expressed target proteins. Cells were treated and extracts analyzed as in (A), using the indicated antibodies. Short (left) and long exposures (right) are shown for each immunoblot. (C) ISG15 modification of p56, an interferon-induced protein. Same as described in (A), with immunoblot analyzed with anti-p56 antibody.

Figure 2

ISG15 Conjugation to Endogenously Versus Exogenously Expressed Target Proteins (A) ISGylation of endogenously expressed target proteins. HEK293T cells were either mock transfected (lane 1); transfected with plasmids expressing Ube1L, UbcH8, Herc5, and ISG15 (lane 2); or transfected with Ube1L, UbcH8, and Herc5 (lane 3). Cell extracts were prepared 48 hr posttransfection and analyzed for ISG15 conjugation by immunoblotting using the indicated antibodies. (B) ISGylation of exogenously expressed target proteins. HEK293T cells were transfected with a plasmid expressing an epitope-tagged target protein (FLAG-IQGAP1 in top two panels, V5-moesin, or V5-Ube1) either alone (lane 1), combined with Ube1L, UbcH8, Herc5, and ISG15 (lane 2), or combined with Ube1L, UbcH8, and Herc5 (lane 3). Cell extracts were prepared 48 hr posttransfection and analyzed by immunoblotting using the indicated antibodies.

Figure 3

ISG15 Conjugation to Proteins Not Previously Identified as ISG15 Targets (A) Modification of human proteins not previously identified as ISG15 targets. HEK293T cells were transfected with plasmids expressing either TAP-E6AP or HA-p53 alone (lane 1), combined with Ube1L, UbcH8, Herc5, and ISG15 (lane 2), or combined with Ube1L, UbcH8, and Herc5 (lane 3). Cell extracts were prepared 48 hr posttransfection and analyzed by immunoblotting with anti-TAP or anti-HA antibodies. See also Figure S2. (B) ISG15 modification to foreign proteins. HEK293T cells were transfected with plasmids expressing the indicated proteins and either V5-HPV18 L1 protein or TAP-tagged Shigella OSPG protein and analyzed as in (A) with anti-V5 or anti-TAP antibodies. (C) Modification of nonoverlapping fragments of IQGAP1. A schematic of IQGAP1 is shown (top), along with three nonoverlapping fragments, A–C, with amino acid numbering shown. Plasmids expressing TAP-tagged fragments A–C were transfected into HEK293T cells and analyzed as in (A) using anti-TAP antibody.

Figure 4

Preferential Modification of Newly Synthesized Proteins (A) Time course of expression and ISGylation of exogenously versus endogenously expressed target proteins. (Left) HEK293T cells were transfected with plasmids expressing either TAP-Ube1 or TAP-IQGAP1 along with Ube1L, UbcH8, and Herc5 (3×). Cell extracts were prepared at the indicated time points after transfection and analyzed by immunoblotting with antibodies against Ube1 or IQGAP1 to compare exogenous and endogenous protein levels (left). (Middle) HEK293T cells were transfected with plasmids expressing TAP-Ube1 or TAP-IQGAP1 plus Ube1L, UbcH8, Herc5, and FLAG-ISG15 (4×). Cell extracts were collected at the indicated time points and analyzed with an antibody against TAP to detect ISGylation of the exogenous targets. Asterisks mark the earliest detectable conjugates to TAP-Ube1 and TAP-IQGAP1. (Right) HEK293T cells were transfected with plasmids expressing Ube1L, UbcH8, Herc5, and FLAG-ISG15 (4×, with no exogenous target protein), and extracts were analyzed for modification of endogenously expressed Ube1 or IQGAP1. (B) Schematic of experimental design: blocking synthesis of an exogenous target protein blocks its ISGylation. A plasmid expressing an epitope-tagged target protein was transfected at time zero; target mRNA (solid line) and target protein (dashed line) accumulated over the next 24 hr. Plasmids expressing Ube1L, UbcH8, Herc5, and FLAG-ISG15 were then transfected, either without or with an siRNA that recognized the target protein mRNA. In the absence of the target siRNA (left), modification of the target protein was expected to occur as the ISG15 conjugation components (double line) accumulated and the target protein continued to be synthesized. In the presence of the target siRNA (right), target protein mRNA would be destroyed as the conjugation components are expressed, but previously synthesized protein persist as a function of their rate of destruction; if continued target protein synthesis is required for ISGylation, then ISGylation of the target protein should not occur. (C) Knockdown of target protein mRNA blocks target protein modification. HEK293T cells were transfected with plasmids expressing either TAP-IQGAP1 or TAP-p53 and transfected again 24 hr later with Ube1L, UbcH8, Herc5, and ISG15, without or with siRNAs against IQGAP1, p53, or an off-target siRNA against hTfr (human transferrin receptor). Cell extracts were prepared 22 hr after the second transfection and analyzed by immunoblotting with anti-TAP antibody.

Figure 5

³⁵S-Cysteine Metabolic Labeling Demonstrates that Pre-Existing Pools of Cellular Proteins Are Not Subject to ISG15 Conjugation (A) Schematic of the experimental design. HEK293T cells were transfected with plasmids expressing either Ube1L, UbcH8, and Herc5 (3×) or Ube1L, UbcH8, Herc5 and either FLAG-ISG15 C78S or FLAG-ΔN-ISG15 (4×). Cells were labeled for 2 hr with ³⁵S -cysteine beginning either 6 or 22 hr posttransfection (gray boxes), corresponding to periods before or during which ISG15 conjugation was occurring (approximate time course of ISG15 conjugation indicated by dashed line). Cell extracts were prepared 24 hr posttransfection. (B) Immunoprecipitation of labeled cellular proteins conjugated to ISG15. ISG15 conjugates were immunoprecipitated with anti-FLAG beads, and ³⁵S-labeled cellular proteins that were conjugated to ISG15 were analyzed by SDS-PAGE and autoradiography. (Lanes 1 and 2) Immunoprecipitation of extracts after 4× transfection (with FLAG-ISG15 C78S), labeled with ³⁵S-cysteine either early (E; 6 hr) or late (L; 22 hr). (Lanes 3 and 4) Immunoprecipitation of extracts after 4× transfection (with FLAG-ΔN-ISG15), labeled with ³⁵S-cysteine either early (E) or late (L). (Lanes 5 and 6) Immunoprecipitation of extracts from untransfected cells, labeled with ³⁵S-cysteine either early (E) or late (L). (C) Cells treated with puromycin contain polypeptides that are modified with both puromycin and ISG15. HEK293T cells were transfected with either four plasmids (FLAG-ISG15, Ube1L, UbcH8, and Herc5) or three plasmids (same, without Ube1L, UbcH8, or Herc5). Twenty-four hours posttransfection, cells were harvested or treated with 1 mM puromycin for 2 min. Cell extracts were prepared, immunoprecipitated with anti-FLAG antibody, and immunoblotted with anti-puromycin antibody.

Figure 6

Herc5 Fractionates with Polysomes (A) Herc5 fractionates with polysomes in HeLa cell extracts. HeLa cells were treated with IFN-β for 24 hr and subjected to polysome analysis (Experimental Procedures). The A₂₅₄ profile shows the positions of the 40S, 60S, 80S, and polysome fractions. Fractions were analyzed by immunoblotting with the indicated antibodies. (B) Herc5 fractionates with the 80S ribosomes after RNase treatment. HeLa cells were treated and analyzed as described in (A); however, RNase was added to the cell lysate after harvest. Fractions were analyzed by immunoblotting with anti-Herc5 antibody. (C) Herc5 fractionates with the 60S ribosomal subunit after EDTA treatment. HeLa cells were treated and analyzed as described in (A), with addition of EDTA to the cell lysate after harvest. Fractions were analyzed by immunoblotting with anti-Herc5 antibody. (D) The RCC1 repeats of Herc5 are required for cosedimentation with polysomes. HEK293T cells were transfected with either HA-Herc5-, HA-ΔRCC-, or HA-Δ100-expressing plasmids and harvested after 24 hr. Cell extracts were fractionated as above and analyzed by anti-HA immunoblotting.

Figure 7

ISGylation of HPV16 L1 Has a Dominant-Negative Effect on Infectivity of HPV Pseudovirus (A) ISG15 Modification of HPV16 L1. 293TT cells were cotransfected with p16shell, alone, or with plasmids expressing ISG15, Ube1L, UbcH8, and Herc5 (4×), or this set of plasmids without ISG15 (3×). Cells extracts were analyzed by SDS-PAGE and immunoblotting with anti-L1 antibody. (B) ISGylated HPV16 L1 is detected in fractions containing HPV pseudovirus. Equal volumes of PsV-containing fractions, prepared in cells expressing active ISGylation enzymes (WT) or inactive mutants (Mut.), were separated by SDS-PAGE and immunoblotted with anti-L1 antibody. (C) ISGylation of HPV16 L1 decreases the infectivity of HPV pseudovirus. 293T cells were infected with equal volumes PsV from the fractions shown in Figure 7B, formed either in the absence (PsV) or presence (PsV + ISG15) of the ISG15 conjugation system. GFP-positive cells were counted 60 hr postinfection by FACS. The infectivity of wild-type PsV was set to 100%. Results are presented both without (middle) and with (right) normalization for total L1 protein concentration. Error bars indicate standard error of the mean of three independent experiments performed in triplicate.