The KCTD family includes tetramerization (T1) domain containing proteins with diverse biological effects. We identified a novel member of the KCTD family, BTBD10. A comprehensive analysis of protein-protein interactions (PPIs) allowed us to put forth a number of testable hypotheses concerning the biological functions for individual KCTD proteins. In particular, we predict that KCTD20 participates in the AKT-mTOR-p70 S6k signaling cascade, KCTD5 plays a role in cytokinesis in a NEK6 and ch-TOG-dependent manner, KCTD10 regulates the RhoA/RhoB pathway. Developmental regulator KCTD15 represses AP-2α and contributes to energy homeostasis by suppressing early adipogenesis. TNFAIP1-like KCTD proteins may participate in post-replication DNA repair through PCNA ubiquitination. KCTD12 may suppress the proliferation of gastrointestinal cells through interference with GABAb signaling. KCTD9 deserves experimental attention as the only eukaryotic protein with a DNA-like pentapeptide repeat domain. The value of manual curation of PPIs and analysis of existing high-throughput data should not be underestimated.

Biofilms, multicellular communities of bacteria, may be an environmental survival and transmission mechanism of Francisella tularensis. Chitinases of F. tularensis ssp. novicida (Fn) have been suggested to regulate biofilm formation on chitin surfaces. However, the underlying mechanisms of how chitinases may regulate biofilm formation are not fully determined. We hypothesized that Fn chitinase modulates bacterial surface properties resulting in the alteration of biofilm formation. We analyzed biofilm formation under diverse conditions using chitinase mutants and their counterpart parental strain. Substratum surface charges affected biofilm formation and initial attachments. Biophysical analysis of bacterial surfaces confirmed that the chi mutants had a net negative-charge. Lectin binding assays suggest that chitinase cleavage of its substrates could have exposed the concanavalin A-binding epitope. Fn biofilm was sensitive to chitinase, proteinase and DNase, suggesting that Fn biofilm contains exopolysaccharides, proteins and extracellular DNA. Exogenous chitinase increased the drug susceptibility of Fn biofilms to gentamicin while decreasing the amount of biofilm. In addition, chitinase modulated bacterial adhesion and invasion of A549 and J774A.1 cells as well as intracellular bacterial replication. Our results support a key role of the chitinase(s) in biofilm formation through modulation of the bacterial surface properties. Our findings position chitinase as a potential anti-biofilm enzyme in Francisella species.

A recently published study by Jeffrey E. Gold et al. (1) presents data that strongly suggest that measles-mumps-rubella (MMR) vaccination negatively correlates with the severity of coronavirus disease 2019 (COVID-19)-related symptoms. Another study by Alba Grifoni et al. (2) demonstrated that antibody titers to spike protein in some unexposed to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) subjects may reach substantial levels, thus suggesting preexisting immunity to the coronavirus. This preexisting immunity may be due to the cross-reactivity with other antigens, for example the ones resulting from previous immunizations. Other studies reported that COVID-19 mortality is higher in countries where Mycobacterium bovis BCG vaccination is not routinely administered (3). Why previously received MMR vaccine may aid in reducing the severity of coronavirus infection symptoms is not clear, but it is tempting to speculate that one or more of the MMR components may be structurally similar to SARS-CoV epitopes recognized by the immune system and may contribute to cross-immunity. Hence, we performed homology analysis between the receptor binding domain (RBD) of the spike protein and the nucleocapsid protein of SARS-CoV-2 to measles, mumps, and rubella proteomes using BLAST (4). A similarity between the RBD of the surface glycoprotein of COVID-causing coronavirus and the measles fusion glycoprotein (chain B) was evident (Fig. 1). The fusion protein of the measles virus is necessary for virus-cell membrane merging and subsequent injection of its ribonucleocapsid complex into the host cell cytosol. More specifically, in fusion glycoprotein (chain B) of attenuated strains of the measles virus of MMR vaccine interacts with the host cell surface receptors, including CD46 (5). Immunogenicity of the fusion glycoprotein is well-known, as it is reported as an effective target for serological responses (6). Similarly, coronavirus RBD is also recognized as an epitope for immune response (7). Our findings support the hypothesis that the chain B of the fusion protein of the measles virus may play a role in anti-SARS-CoV-2 immunological responses by its cross-reaction with RBD protein of the COVID-19-causing virus. An alternative explanation to the MMR-induced alleviation of coronavirus infection is that the RBD of coronaviral S protein may weakly engage the receptor for measles virus CD46, a complement regulatory molecule, also known as membrane cofactor protein MCP (8). It is tempting to speculate that anti-measles virus antibodies may bind SARS-CoV-2 in a way that prevents it from interacting with CD46 receptor, which normally protects the cells against complement-mediated cell lysis and, therefore, alleviates COVID-related abnormal activation of the complement posed in some recent studies (9-11). Notably, only attenuated measles vaccine strains recognize CD46, while wild-type disease-causing virus employs other entry (8), possibly explaining why the anti-COVID protective effects are detected after MMR vaccination but no...

The low-density lipoprotein receptor (LDLR) family of receptors are cell-surface receptors that internalize numerous ligands and play crucial role in various processes, such as lipoprotein metabolism, hemostasis, fetal development, etc. Previously, receptor-associated protein (RAP) was described as a molecular chaperone for LDLR-related protein 1 (LRP1), a prominent member of the LDLR family. We aimed to verify this role of RAP for LRP1 and two other LDLR family receptors, LDLR and vLDLR, and to investigate the mechanisms of respective interactions using a cell culture model system, purified system, and in silico modelling. Upon coexpression of RAP with clusters of the ligand-binding complement repeats (CRs) of the receptors in secreted form in insect cells culture, the isolated proteins had increased yield, enhanced folding, and improved binding properties compared with proteins expressed without RAP, as determined by circular dichroism and surface plasmon resonance. Within LRP1 CR-clusters II and IV, we identified multiple sites comprised of adjacent CR doublets, which provide alternative bivalent binding combinations with specific pairs of lysines on RAP. Mutational analysis of these lysines within each of isolated RAP D1/D2 and D3 domains having high affinity to LRP1 and of conserved tryptophans on selected CR-doublets of LRP1, as well as in silico docking of a model LRP1 CR-triplet with RAP, indicated a universal role for these residues in interaction of RAP and LRP1. Consequently, we propose a new model of RAP interaction with LDLR family receptors based on switching of the bivalent contacts between molecules over time in a dynamic mode.

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