Being a major first line of immune defense, the complement system keeps a constant vigil against viruses. Its ability to recognize large panoply of viruses and virus-infected cells, and trigger the effector pathways, results in neutralization of viruses and killing of the infected cells. This selection pressure exerted by complement on viruses has made them evolve a multitude of countermeasures. These include targeting the recognition molecules for the avoidance of detection, targeting key enzymes and complexes of the complement pathways like C3 convertases and C5b-9 formation – either by encoding complement regulators or by recruiting membrane-bound and soluble host complement regulators, cleaving complement proteins by encoding protease, and inhibiting the synthesis of complement proteins. Additionally, viruses also exploit the complement system for their own benefit. For example, they use complement receptors as well as membrane regulators for cellular entry as well as their spread. Here, we provide an overview on the complement subversion mechanisms adopted by the members of various viral families including Poxviridae, Herpesviridae, Adenoviridae, Flaviviridae, Retroviridae, Picornaviridae, Astroviridae, Togaviridae, Orthomyxoviridae and Paramyxoviridae.

Autosomal recessive mutations in the PINK1 gene are causal for Parkinson's disease (PD). PINK1 encodes a mitochondrial localized protein kinase that is a master-regulator of mitochondrial quality control pathways. Structural studies to date have elaborated the mechanism of how mutations located within the kinase domain disrupt PINK1 function; however, the molecular mechanism of PINK1 mutations located upstream and downstream of the kinase domain is unknown. We have employed mutagenesis studies to define the minimal region of human PINK1 required for optimal ubiquitin phosphorylation, beginning at residue Ile111. Inspection of the AlphaFold human PINK1 structure model predicts a conserved N-terminal α-helical extension (NTE) domain forming an intramolecular interaction with the C-terminal extension (CTE), which we corroborate using hydrogen/deuterium exchange mass spectrometry of recombinant insect PINK1 protein. Cell-based analysis of human PINK1 reveals that PD-associated mutations (e.g. Q126P), located within the NTE : CTE interface, markedly inhibit stabilization of PINK1; autophosphorylation at Serine228 (Ser228) and Ubiquitin Serine65 (Ser65) phosphorylation. Furthermore, we provide evidence that NTE and CTE domain mutants disrupt PINK1 stabilization at the mitochondrial Translocase of outer membrane complex. The clinical relevance of our findings is supported by the demonstration of defective stabilization and activation of endogenous PINK1 in human fibroblasts of a patient with early-onset PD due to homozygous PINK1 Q126P mutations. Overall, we define a functional role of the NTE : CTE interface towards PINK1 stabilization and activation and show that loss of NTE : CTE interactions is a major mechanism of PINK1-associated mutations linked to PD.

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Regulation of complement activation in the host cells is mediated primarily by the regulators of complement activation (RCA) family proteins that are formed by tandemly repeating complement control protein (CCP) domains. Functional annotation of these proteins, however, is challenging as contiguous CCP domains are found in proteins with varied functions. Here, by employing an in silico approach, we identify five motifs which are conserved spatially in a specific order in the regulatory CCP domains of known RCA proteins. We report that the presence of these motifs in a specific pattern is sufficient to annotate regulatory domains in RCA proteins. We show that incorporation of the lost motif in the fourth long-homologous repeat (LHR-D) in complement receptor 1 regains its regulatory activity. Additionally, the motif pattern also helped annotate human polydom as a complement regulator. Thus, we propose that the motifs identified here are the determinants of functionality in RCA proteins.

The complement system is highly efficient in targeting pathogens, but lack of its apposite regulation results in host-cell damage, which is linked to diseases. Thus, complement activation is tightly regulated by a series of proteins, which primarily belong to the regulators of complement activation (RCA) family. Structurally, these proteins are composed of repeating complement control protein (CCP) domains where two to four successive domains contribute to the regulatory functions termed decay-accelerating activity (DAA) and cofactor activity (CFA). However, the precise constitution of the functional units and whether these units can be joined to form a larger composition with dual function have not been demonstrated. Herein, we have parsed the functional units for DAA and CFA by constructing chimeras of the decay-accelerating factor (DAF) that exhibits DAA and membrane cofactor protein (MCP) that exhibits CFA. We show that in a four-CCP framework, a functional unit for each of the regulatory activities is formed by only two successive CCPs wherein each participates in the function, albeit CCP2 has a bipartite function. Additionally, optimal activity requires C-terminal domains that enhance the avidity of the molecule for C3b/C4b. Furthermore, by composing a four-CCP DAF-MCP chimera with robust CFA (for C3b and C4b) and DAA (for classical and alternative pathway C3 convertases), named decay cofactor protein, we show that CCP functional units can be linked to design a dual-activity regulator. These data indicate that the regulatory determinants for these two biological processes are distinct and modular in nature. complement | complement regulation | RCA proteins | decay-accelerating factor | membrane cofactor protein Author contributions: H.

Viruses are obligate parasites of cellular hosts and therefore are constantly confronted with the host immune system. Evasion of innate immunity mechanisms by viruses is paramount for the establishment of their infection. The complement system can directly neutralize viruses and also augments adaptive immune responses against them. This system, therefore, is central to host innate immune surveillance, and viruses have evolved a multitude of ways to escape its assault. A major strategy employed by viruses is the molecular mimicry of human complement regulators, namely regulators of complement activation (RCA) proteins and CD59. Herein, we outline up‐to‐date information on the structure, function and role of viral homologs of the human complement regulators in viral pathogenesis.

Among the two GroEL paralogs in Mycobacterium tuberculosis, GroEL1 and GroEL2, GroEL1 has a characteristic histidine-rich C terminus. Since histidine richness is likely to be involved in metal binding, we attempted to decipher the role of GroEL1 in chelating metals and the consequence on M. tuberculosis physiology. Isothermal titration calorimetry showed that GroEL1 binds copper and other metals. Mycobacterial viability assay, redox balance, and DNA protection assay concluded that GroEL1 protects from copper stress in vitro. Solution X-ray scattering and constrained modeling of GroEL1 À/+ copper ions showed reorientation of the apical domain as seen in functional assembly. We conclude that the duplication of chaperonin genes in M. tuberculosis might have led to their evolutionary divergence and consequent functional divergence of chaperonins.

Mycobacterial GroELs namely GroEL1 and GroEL2 belong to the family of molecular chaperones, chaperonins. Chaperonins in Escherichia coli are termed as GroEL and GroES which are encoded by essential genes and are involved in cellular protein folding. GroEL1 has a characteristic Histidine-rich C-terminus contrary to its essential paralog GroEL2 and E. coli GroEL which have hydrophobic (GGM) repeats. Since Histidine richness is likely to be involved in metal binding, in this study we have attempted to decipher the role of GroEL1 protein in chelating metals and the consequent role on M. tuberculosis physiology. Using isothermal titration calorimetry (ITC), we found that GroEL1 binds copper, nickel and cobalt, with the highest binding affinity to copper. Since copper is known to be toxic at higher concentration, we cultured Wild Type M. tuberculosis H37Rv, groEL1 knock-out and groEL1-complemented strain with increasing concentrations of copper. We found that M. tuberculosis groEL1 knock out strain is more sensitive to copper than the wild type. Further hypothesizing that the probable mode of action of copper is by induction of oxidative stress, we attempted to understand the role of GroEL1 in redox silencing and hydroxyl radical mediated DNA damage. We interestingly found through our in vitro studies that GroEL1 is helpful in protection from copper stress through maintaining redox balance and free radical mediated DNA damage. Thus, these results indicate that the duplication of chaperonin genes in M. tuberculosis might have led to their evolutionary divergence and resulted in a functional divergence of chaperonins.