Post-translational modification of glutamine and lysine residues of HIV-1 aspartyl protease by transglutaminase increases its catalytic activity

Cell surface transglutaminase required for nodavirus entry into freshwater prawn hemocytes

To identify molecules involved in Macrobrachium rosenbergii nodavirus (MrNV) entry into hemocytes of the giant freshwater prawn M. rosenbergii, biotinylated prawn hemocyte membrane proteins were prepared, purified and separated by SDS-PAGE. The proteins were blotted on the nitrocellulose membrane before incubation with the MrNV capsid protein (MrNV-CP) by a VOPBA technique. Subsequent mass spectrometry and analysis of immune-reactive bands represent putative binding partners including transglutaminase (TG), actin, α2-macroglobulin, α1-tubulin, F₁-ATP synthase β-subunit and a currently uncharacterized protein. The sequence of TG has been characterized and found 5 amino acids differences to a previously reported MrTG (ADX99580), mainly at its N-terminal part and thus, we named it MrTGII (KM008611). Recombinant MrTGII was prepared to produce a polyclonal antibody against it, which was successfully revealed the presence of MrTGII (100 kDa) in prawn hemocyte lysates. Using the pentylamine-biotin incorporation assay, an acyl transfer reaction was observed when hemocyte lysates were added to solutions containing MrNV-CP, suggesting that hemocyte MrTG could use MrNV-CP as the substrate. The expression levels of MrTGII were changed during the course of MrNV infection. By using immunostaining technique, location of MrTGII on the hemocyte surface was confirmed. Specific interaction between MrTGII with MrNV-CP in a dose-dependent manner was confirmed by in vitro ELISA assay. The highest binding activity of MrNV-CP was found with the N-terminal portion of the protein. In vitro neutralization using anti-MrTGII antibody resulted in inhibition of MrNV attachment to the hemocyte surface, accompanied by a dramatic reduction in viral replication. This is the first time that crustacean TG has been shown to be involved in viral entry, in addition to its roles in blood clotting and haematopoiesis.

Uncovering protein polyamination by the spermine-specific antiserum and mass spectrometric analysis

The polyamines spermidine and spermine, and their precursor putrescine, have been shown to play an important role in cell migration, proliferation, and differentiation. Because of their polycationic property, polyamines are traditionally thought to be involved in DNA replication, gene expression, and protein translation. However, polyamines can also be covalently conjugated to proteins by transglutaminase 2 (TG2). This modification leads to an increase in positive charge in the polyamine-incorporated region which significantly alters the structure of proteins. It is anticipated that protein polyamine conjugation may affect the protein-protein interaction, protein localization, and protein function of the TG2 substrates. In order to investigate the roles of polyamine modification, we synthesized a spermine-conjugated antigen and generated an antiserum against spermine. In vitro TG2-catalyzed spermine incorporation assays were carried out to show that actin, tubulins, heat shock protein 70 and five types of histone proteins were modified with spermine, and modification sites were also identified by liquid chromatography and linear ion trap-orbitrap hybrid mass spectrometry. Subsequent mass spectrometry-based shotgun proteomic analysis also identified 254 polyaminated sites in 233 proteins from the HeLa cell lysate catalyzed by human TG2 with spermine, thus allowing, for the first time, a global appraisal of site-specific protein polyamination. Global analysis of mouse tissues showed that this modification really exists in vivo. Importantly, we have demonstrated that there is a new histone modification, polyamination, in cells. However, the functional significance of histone polyamination demands further investigations.

Posttranslational modifications of HIV-1 integrase by various cellular proteins during viral replication

HIV-1 integrase (IN) is a key viral enzyme during HIV-1 replication that catalyzes the insertion of viral DNA into the host genome. Recent studies have provided important insights into the multiple posttranslational modifications (PTMs) of IN (e.g., ubiquitination, SUMOylation, acetylation and phosphorylation), which regulate its multifaceted functions. A number of host cellular proteins, including Lens Epithelium‑derived Growth factor (LEDGF/p75), p300 and Ku70 have been shown to interact with IN and be involved in the PTM process of IN, either facilitating or counteracting the IN PTMs. Although previous studies have revealed much about the important roles of IN PTMs, how IN functions are fine-tuned by these PTMs under the physiological setting still needs to be determined. Here, we review the advances in the understanding of the mechanisms and roles of multiple IN PTMs.

Cellular functions of tissue transglutaminase

Transglutaminase 2 (TG2 or tissue transglutaminase) is a highly complex multifunctional protein that acts as transglutaminase, GTPase/ATPase, protein disulfide isomerase, and protein kinase. Moreover, TG2 has many well-documented nonenzymatic functions that are based on its noncovalent interactions with multiple cellular proteins. A vast array of biochemical activities of TG2 accounts for its involvement in a variety of cellular processes, including adhesion, migration, growth, survival, apoptosis, differentiation, and extracellular matrix organization. In turn, the impact of TG2 on these processes implicates this protein in various physiological responses and pathological states, contributing to wound healing, inflammation, autoimmunity, neurodegeneration, vascular remodeling, tumor growth and metastasis, and tissue fibrosis. TG2 is ubiquitously expressed and is particularly abundant in endothelial cells, fibroblasts, osteoblasts, monocytes/macrophages, and smooth muscle cells. The protein is localized in multiple cellular compartments, including the nucleus, cytosol, mitochondria, endolysosomes, plasma membrane, and cell surface and extracellular matrix, where Ca(2+), nucleotides, nitric oxide, reactive oxygen species, membrane lipids, and distinct protein-protein interactions in the local microenvironment jointly regulate its activities. In this review, we discuss the complex biochemical activities and molecular interactions of TG2 in the context of diverse subcellular compartments and evaluate its wide ranging and cell type-specific biological functions and their regulation.

Transglutaminase-mediated internal protein labeling with a designed peptide loop

Post-translational internal protein labeling was explored through the insertion of a 13-mer peptidyl loop specifically recognized by microbial transglutaminase (MTG). The peptidyl loop included one lysine residue (abbreviated as the K-loop), and was designed and inserted into two different regions of the protein bacterial alkaline phosphatase (BAP). MTG-mediated selective labeling of a lysine residue in the K-loop was achieved with a functional Gln-donor substrate. Internal protein labeling in the vicinity of the active site of BAP (residues 91-93) markedly decreased the activity of the enzyme. Conversely, insertion of the K-loop at a site distal from the active site (residues 219-221) afforded site-specific and covalent internal protein labeling without impairing the activity of the enzyme.