Elucidating the Mechanism Underlying UBA7•UBE2L6 Disulfide Complex Formation

Reviewer #1 (Public review):

Summary:

Chen and colleagues describe mechanisms by which UBA7 and UBE2L6 form disulfide bonds, disrupting the ISG15 transfer cascade. As other similar structures are currently available, the authors further note that the spontaneous formation of this disulfide suggests that it is a potential regulatory mechanism. Demonstrating that this mechanism occurs and is modulated in cells would greatly improve the impact of their work.

Strengths:

The various biochemical and structural experiments are largely convincing.

Weaknesses:

(1) The main point of the paper is that this covalent complex could occur and is potentially regulated in cells is limited. The authors even show an experiment in cells where this complex is formed by expressing UBE2L6-V5 and GFP-UBA7, awkwardly referenced in the discussion.

The authors should consider attempting an experiment with endogenous proteins and either modulate the formation of this complex in different cellular conditions or downplay this part of their story. For example, this sentence, "This redox-sensitive complex implies a link between oxidative stress and regulation of the immune response, highlighting a potential therapeutic target for modulating immune reactions arising from infections and inflammatory conditions." is in the abstract and should be excluded or rephrased considering the lack of cellular data.

Also, their one-cell-based experiment is shown in the discussion. This should be in the results as is standard practice but also repeated. It appears that the reduced lanes don't seem to have GFP or the GFP-UBA7. Without those controls, this experiment seems incomplete.

(2) Their intro sets up the paper to explain the disulfide formation they see in Figure 1, but a more fitting experiment would be to look at the disulfide formation between UBA7 and UBE2L6 at different pHs. It would nicely supplement the biochemical pKa data as this reaction is their focal point.

(3) While the biochemical data is extensive, it is not concise or easily accessible to a broad readership. The authors should try to clarify and simplify the text overall. Furthermore, many figure callouts are missing, interfering with the clarity of the text.

Minor

(1) Because the experiments are pKa dependent, knowing what buffers the proteins were finished in (final SEC purification step) is important. Similarly - for all assays, the buffers were not reported (SEC-MALS, biochemical assays).

(2) While the CBB and fluorescent gel assays look convincing, more controls are needed for their SEC experiments (Figure 1d), particularly because the authors definitively say the binding is because of S-S bonds. Using a reducing buffer like TCEP or DTT or their catalytic mutants to show reduced co-migration would be helpful. This is even more important given the reported high affinities between UBA7/UBE2L6 in Figure 6.

(3) Based on the data presented, it is unclear that the kinetic values are taken within initial velocity regimes. Some data in the supplement showing that the single time points represent initial velocities would be appreciated.

(4) As stated, "Previous experiments reveal an intriguing anomaly during the UBA7-UBE2L6-ISG15 thioester transfer reaction. Despite adding more ISG15 and UBE2L6, the level of UBE2L6~ISG15 remained the same." This experiment should be shown or the statement removed.

(5) Similarly, "Forty human E2 enzymes are classified in the InterProdatabase (https://www.ebi.ac.uk/interpro/), with the majority interacting with UBA1, whereas UBE2L6 and UBE2Z exclusively interact with UBA7 and UBA6, respectively." Is missing a reference.

Reviewer #2 (Public review):

Summary:

Chen et al. describe by different techniques that UBA7 and UBE2L6 readily form a complex that is covalently linked by a disulfide bond involving the active site cysteines of UBA6 and UBE2L6. Furthermore, they determined cryo-EM structures of the disulfide-linked UBY7-UBE2L6 complex in the absence and presence of ISG15. They propose that this disulfide-linked complex blocks ISGylation by temporarily rendering UBA7 inactive.

Strengths:

The authors employ a wide variety of techniques to study the formation of the binary Uba7-UBE2L6 and ternary UBA7-UBE2L6-ISG15 complexes including the structural characterization of the two complexes by cryo-EM. Despite the shortcomings (see below), the authors provide numerous valuable data that characterize the first steps of the ISGylation pathway, namely the activation of ISG15 and its transfer to UBE2L6.

Weaknesses:

(1) The authors correctly state that "Immune responses often entail the generation or reactive oxygen species, antioxidant defense mechanisms, and redox signaling" (1st sentence of 3rd paragraph in the Introduction). Based on the data presented in this study these cellular responses should lead to the formation of the covalent UBA7-UBE2L6. Since this complex renders UBA7 inactive, thus preventing it from initiating the ISGylation cascade in response to viral infections, the underlying cellular logic of complex formation remains a mystery.

The bulk of their work describes in vitro experiments, which will certainly not reflect the in vivo situation and hence one cannot rule out that this complex will not form inside cells. The authors have also observed this complex in HEK293T cells, however, this involved overexpression of both proteins and one can thus not rule out that the disulfide-linked complex will not form at physiological protein levels. Furthermore, this cellular model appears not to be a suitable system.

(2) The authors carried out a comparative analysis of E1-E2 disulfide bond formation with UBA1, the major activating enzyme for ubiquitin, and UBE2L3, a ubiquitin-specific E2. The choice of UBE2L3 was motivated by its close relationship to UBE2L6. From these studies, the authors conclude that UBA1 does not form the corresponding complex. Given that there are over 30 ubiquitin-specific E2s this conclusion does not rest on a very solid basis, since, as demonstrated for example in this study (PMID: 22949505), at least yeast Uba1 forms a disulfide-linked complex with Cdc34. Another study documenting the formation of a disulfide-linked complex between Uba1 and an E2 enzyme, in this case, Rad6, (PMID: 35613580) is even cited by the authors. If the authors want to make the argument that Uba1 does not form corresponding E1-E2 complexes, they need to repeat their experiments with a representative panel of human E2 enzymes and the two enzymes employed in the aforementioned studies (Cdc34 and Rad6) or, more precisely, their human counterparts represent obvious starting points. Depending on the outcome of these studies the experiments with the CCL mutants need to be revisited.

Reviewer #3 (Public review):

Summary:

In this manuscript, "Elucidating the mechanism underlying UBA7-UBE2L6 disulfide complex formation", Chen et al. describe the mechanism of spontaneous disulfide bond formation between the active site cysteines of UBA7 and UBE2L6. Employing state-of-the-art biochemistry, cryo-EM, and HDX mass spec approaches, the authors provide insights into how this mechanism occurs in UBA7/UBE2L6 but not in related ubiquitin enzymes. A central conclusion of the study is that the length of the catalytic cysteine loop (CCL) in UBA7 is insufficient to block access to the E1's catalytic cysteine, thereby facilitating UBE2L6 disulfide formation. In contrast, the CCL of UBA1 is sufficiently long and shields its catalytic cysteine, preventing access to the Ub E2 enzymes. In addition to the CCL, the authors also show that UBA7's specificity and strong binding affinity for UBE2L6 help promote this disulfide-linked E1-E2 complex.

Strengths:

The data within in manuscript is interesting and significantly contributes to our understanding of the mechanisms of the ISG15 conjugation pathway. Moreover, the biochemical and structural experiments were performed at an exceptionally high level and the data throughout the manuscript is convincing.

Weaknesses:

It is not clear whether this regulatory mechanism occurs in a biological context (e.g., during IFN signaling or oxidative stress). However, this weakness is somewhat offset by the last experiment of the manuscript which demonstrates the existence of UBA7-UBE2L6 disulfide complex formation in cells under overexpression conditions. If the authors could expand upon this finding, as outlined below it would further improve their study.