ZC3HC1 Is a Novel Inherent Component of the Nuclear Basket, Resident in a State of Reciprocal Dependence with TPR

The nuclear basket (NB) scaffold, a fibrillar structure anchored to the nuclear pore complex (NPC), is regarded as constructed of polypeptides of the coiled-coil dominated protein TPR to which other proteins can bind without contributing to the NB’s structural integrity. Here we report vertebrate protein ZC3HC1 as a novel inherent constituent of the NB, common at the nuclear envelopes (NE) of proliferating and non-dividing, terminally differentiated cells of different morphogenetic origin. Formerly described as a protein of other functions, we instead present the NB component ZC3HC1 as a protein required for enabling distinct amounts of TPR to occur NB-appended, with such ZC3HC1-dependency applying to about half the total amount of TPR at the NEs of different somatic cell types. Furthermore, pointing to an NB structure more complex than previously anticipated, we discuss how ZC3HC1 and the ZC3HC1-dependent TPR polypeptides could enlarge the NB’s functional repertoire.

Proteome-scale Analysis of Vertebrate Protein Thermoadaptation Modulated by Dynamic Allostery and Protein Solvation

Despite large differences in behaviors and living conditions, vertebrate organisms share the great majority of proteins often with subtle differences in amino acid sequence. By comparing a set of substantially homologous proteins between model vertebrate organisms at a sub-proteome level, we discover a pattern of amino acid conservation and a shift in amino acid use, noticeably with an apparent distinction between homeotherms (warm-blooded species) and poikilotherms (cold-blooded species). Importantly, we establish a connection between the thermoadaptation of protein sequences manifest in the evolved proteins and two of their physical features: a change in their proteins dynamics and in their solvation. For poikilotherms such as frog and fish, the lower body temperature is expected to increase the association of proteins due to a decrease in protein dynamics and correspondingly lower entropy penalty on binding. In order to prevent overly-sticky protein association at low temperatures, we find that poikilotherms enhance the solvation of their proteins by favoring polar amino acids on their protein’s surface. This study unveils a general mechanism behind amino acid choices that constitute part of the thermoadaptation of vertebrate organisms at the molecular level.

Metaxin 3 is a Highly Conserved Vertebrate Protein Homologous to Mitochondrial Import Proteins and GSTs

ABSTRACTMetaxin 3 genes are shown to be widely conserved in vertebrates, including mammals, birds, fish, amphibians, and reptiles. Metaxin 3 genes, however, are not found in invertebrates, plants, and bacteria. The predicted metaxin 3 proteins were identified by their homology to the metaxin 3 proteins encoded by zebrafish and Xenopus cDNAs. Further evidence that they are metaxin proteins was provided by the presence of GST_N_Metaxin, GST_C_Metaxin, and Tom37 protein domains, and the absence of other major domains. Alignment of human metaxin 3 and human metaxin 1 predicted amino acid sequences showed 45% identities, while human metaxin 2 had 23% identities. These results indicate that metaxin 3 is a distinct metaxin. A wide variety of vertebrate species—including human, zebrafish, Xenopus, dog, shark, elephant, panda, and platypus—had the same genes adjacent to the metaxin 3 gene. In particular, the thrombospondin 4 gene (THBS4) is next to the metaxin 3 gene (MTX3). By comparison, the thrombospondin 3 gene (THBS3) is next to the metaxin 1 gene (MTX1). Phylogenetic analysis showed that metaxin 3, metaxin 1, and metaxin 2 protein sequences formed separate clusters, but with all three metaxins being derived from a common ancestor. Alpha-helices dominate the predicted secondary structures of metaxin 3 proteins. Little beta-strand is present. The pattern of 9 helical segments is also found for metaxins 1 and 2.

Positive selection in dNTPase SAMHD1 throughout mammalian evolution

The vertebrate protein SAMHD1 is highly unusual in having roles in cellular metabolic regulation, antiviral restriction, and regulation of innate immunity. Its deoxynucleoside triphosphohydrolase activity regulates cellular dNTP concentration, reducing levels below those required by lentiviruses and other viruses to replicate. To counter this threat, some primate lentiviruses encode accessory proteins that bind SAMHD1 and induce its degradation; in turn, positive diversifying selection has been observed in regions bound by these lentiviral proteins, suggesting that primate SAMHD1 has coevolved to evade these countermeasures. Moreover, deleterious polymorphisms in humanSAMHD1are associated with autoimmune disease linked to uncontrolled DNA synthesis of endogenous retroelements. Little is known about how evolutionary pressures affect these different SAMHD1 functions. Here, we examine the deeper history of these interactions by testing whether evolutionary signatures in SAMHD1 extend to other mammalian groups and exploring the molecular basis of this coevolution. Using codon-based likelihood models, we find positive selection in SAMHD1 within each mammal lineage for which sequence data are available. We observe positive selection at sites clustered around T592, a residue that is phosphorylated to regulate SAMHD1 activity. We verify experimentally that mutations within this cluster affect catalytic rate and lentiviral restriction, suggesting that virus–host coevolution has required adaptations of enzymatic function. Thus, persistent positive selection may have involved the adaptation of SAMHD1 regulation to balance antiviral, metabolic, and innate immunity functions.

The missing link: allostery and catalysis in the anti-viral protein SAMHD1

Vertebrate protein SAMHD1 (sterile-α-motif and HD domain containing protein 1) regulates the cellular dNTP (2′-deoxynucleoside-5′-triphosphate) pool by catalysing the hydrolysis of dNTP into 2′-deoxynucleoside and triphosphate products. As an important regulator of cell proliferation and a key player in dNTP homeostasis, mutations to SAMHD1 are implicated in hypermutated cancers, and germline mutations are associated with Chronic Lymphocytic Leukaemia and the inflammatory disorder Aicardi–Goutières Syndrome. By limiting the supply of dNTPs for viral DNA synthesis, SAMHD1 also restricts the replication of several retroviruses, such as HIV-1, and some DNA viruses in dendritic and myeloid lineage cells and resting T-cells. SAMHD1 activity is regulated throughout the cell cycle, both at the level of protein expression and post-translationally, through phosphorylation. In addition, allosteric regulation further fine-tunes the catalytic activity of SAMHD1, with a nucleotide-activated homotetramer as the catalytically active form of the protein. In cells, GTP and dATP are the likely physiological activators of two adjacent allosteric sites, AL1 (GTP) and AL2 (dATP), that bridge monomer–monomer interfaces to stabilise the protein homotetramer. This review summarises the extensive X-ray crystallographic, biophysical and molecular dynamics experiments that have elucidated important features of allosteric regulation in SAMHD1. We present a comprehensive mechanism detailing the structural and protein dynamics components of the allosteric coupling between nucleotide-induced tetramerization and the catalysis of dNTP hydrolysis by SAMHD1.