Current Protein and Peptide Science - Volume 4, Issue 1, 2003

Poly-N-acetyllactosamine structures occur in mammalian glycoproteins in both N- and O-linked glycans. They represent a backbone for additional modifications by fucosyltransferases, sialyltransferases and sulfotransferases. These glycans have been suggested to be involved in biospecific interactions with selectins and other glycan-binding proteins. Moreover, the poly-Nacetyllactosamine chains in N-glycans have been found to promote tumor progression and metastasis. Poly-N-acetyllactosamine chains are synthesized by repeated alternating additions of Nacetylglucosamine and galactose, catalyzed by β-1,3-N-acetylglucosaminyltransferases (poly-N-acetyllactosamine synthase) and β-1,4-galactosyltransferases. This review describes the poly-N-acetyllactosamine assembling machinery and focuses on recent advances in the molecular cloning and characterization of poly-N-acetyllactosamine synthase gene families. Recent progress in revealing the biological functions of poly-N-acetyllactosamine structures by various approaches in vitro and in vivo using different model systems has also been summarized.

Many biologically important interactions occur between proteins and carbohydrates. The examination of these interactions at the atomic level is critical not only in understanding the nature of these interactions and their biological role, but also in the design of effective modulators of these interactions. While experimentally obtained structural information is preferred, quite often this information is unavailable. In order to address this, several methods have been developed to probe the interactions between protein and carbohydrate in the absence of structural data. These methods map the interactions between protein and carbohydrate, and identify the groups involved, both at the carbohydrate and protein level. Here, we review these developments, and examine the strengths, weaknesses, and pitfalls of these methods.

As a result of increasing drug resistance in pathogenic bacteria, there is a critical need for novel broad-spectrum antibacterial agents. As fatty acid synthesis (FAS) in bacteria is an essential process for cell survival, the enzymes involved in the FAS pathway have emerged as promising targets for antimicrobial agents. Several lines of evidence have indicated that bacterial condensing enzymes are central to the initiation and elongation steps in bacterial fatty acid synthesis and play a pivotal role in the regulation of the entire fatty acid synthesis pathway. β-ketoacyl-acyl carrier protein (ACP) synthases (KAS) from various bacterial species have been cloned, expressed and purified in large quantities for detailed enzymological, structural and screening studies. Availability of purified KAS from a variety of bacteria, along with a combination of techniques, including combinatorial chemistry, high-throughput screening, and rational drug design based on crystal structures, will undoubtedly aid in the discovery and development of much needed potent and broadspectrum antibacterial agents. In this review we summarize the biochemical, biophysical and inhibition properties of β-ketoacyl-ACP synthases from a variety of bacterial species.

Biosynthetic pathways for the formation of neuroactive peptides and the processes for their inactivation include several enzymatic steps. In addition to enzymatic processing and degradation, several neuropeptides have been shown to undergo enzymatic conversion to fragments with retained or modified biological activity. This has most clearly been demonstrated for e.g. opioid peptides, tachykinins, calcitonin gene-related peptide (CGRP) as well as for peptides belonging to the renin-angiotensin system. Sometimes the released fragment shares the activity of the parent compound. However, in many cases the conversion reaction is linked to a change in the receptor activation profile, i.e. the generated fragment acts on and stimulates a receptor not recognized by the parent peptide. This review will describe the characteristics of certain neuropeptide fragments having the ability to modify the biological action of the peptide from which they are derived. Focus will be directed to the tachykinins, the opioid peptides, angiotensins as well as to CGRP, bradykinin and nociceptin. The κ opioid receptor selective opioid peptide, dynorphin, recognized for its ability to produce dysphoria, is converted to the δ opioid receptor agonist Leu-enkephalin, with euphoric properties. The tachykinins, typified by substance P (SP), is converted to the bioactive fragment SP(1-7), a heptapeptide mimicking some but opposing other effects of the parent peptide. The bioactive angiotensin II, known to bind to and stimulate the AT-1 and AT-2 receptors, is converted to angiotensin IV (i.e. angiotensin 3-8) with preference for the AT-4 sites or to angiotensin (1-7), not recognized by any of these receptors. Both angiotensin IV and angiotensin (1-7) are biologically active. For example angiotensin (1-7) retains some of the actions ascribed for angiotensin II but is shown to counteract others. Thus, it is obvious that the activity of many neuroactive peptides is modulated by bioactive fragments, which are formed by the action of a variety of peptidases. This phenomenon appears to represent an important regulatory mechanism that modulates many neuropeptide systems but is generally not acknowledged.

Neuropeptide Y (NPY) and the related receptors represent a widely diffused system that is involved in the regulation of multiple biological functions. NPY, a 36-aminoacid peptide expressed in several areas of the nervous system, is a pleiotropic factor participating to the control of some physiological processes, such as cognitive functions, eating behavior, circadian rhythms, neuroendocrine mechanisms, reproductive and cardiovascular functions. NPY acts through a series of G-protein-associated membrane receptors (NPY-Rs), characterized by different tissue distribution and affinity for the ligand.The expression and secretion of NPY and the expression of NPY-R isoforms are controlled by a very wide range of agents, acting in an endocrine and / or paracrine fashion. NPY and NPY-Rs appear to be strongly involved in the control of eating behavior, their expression is modulated by changes of food intake and energy balance and is disrupted in several animal models of obesity and diabetes. Moreover, the hypothalamic NPY system appears to integrate signals of energy balance in the modulation of the reproductive axis. Agents that stimulate their expression include activators of intracellular signalling pathways (protein kinase A and C), classical neurotransmitters, steroid and peptide hormones and growth factors, while other agents (leptin, insulin and retinoic acid) have been shown to be inhibitory. Interestingly, some agents, like retinoic acid, have been shown to modulate the expression of both NPY and NPY-Rs in the same direction, thus providing a fine mechanism for the tuning of the system.The regulation of NPY / NPY-R expression and function appears to be part of a complex system controlling multiple physiological functions, and its disruption might be relevant in the pathophysiology of disease states such as obesity.

ES-62 is a major secreted glycoprotein of the rodent filarial nematode Acanthocheilonema viteae and homologue of molecules found in filarial nematodes which parasitise humans. The molecule consists of a tetramer of apparently identical monomers of ∼62 kDa which we have shown by sedimentation equilibrium analytical ultracentrifugation to strongly associate. ES-62 is one of several filarial nematode proteins to contain the unusual posttranslational modification of phosphorylcholine (PC) addition. Specifically, we have found that PC is attached to one of three distinct N-type glycans we have characterised on the molecule. The amino acid sequence of ES-62 shows 37-39%identity with a family of 6 other proteins, some of which have been predicted to be amino- or carboxy-peptidases. We have also found that ES-62 is able to interact with a number of cells of the immune system, specifically B- and Tlymphocytes, macrophages and dendritic cells. Lymphocytes exposed to ES-62 in vitro or in vivo are less able to proliferate in response to ligation via the antigen receptor. Peritoneal macrophages pre-exposed to the molecule are less able to produce the cytokines IL-12, IL-6 and TNF-α following subsequent incubation with the classical stimulators IFNγ and LPS. Dendritic cells allowed to mature in the presence of ES-62 acquire a phenotype, which allows them to induce anti-inflammatory “TH2-type” responses. With respect to immunomodulation, the PC moiety of the parasite molecule appears to be predominantly responsible for the effects on lymphocyte proliferation at least and we have also found that its removal converts the murine IgG antibody response to ES-62 from solely IgG1 to mixed IgG1 / IgG2a. ES-62 appears to interact with cells of the immune system in a PC-dependent manner and, at least in part, via a molecule of ∼82 kDa. Studies of the interaction in lymphocytes show that it is associated with activation of certain signal transduction molecules including a number of protein tyrosine kinases and mitogen activated protein kinases (MAPkinases). Although such activation is insufficient to induce proliferation, it serves to almost completely desensitise the cells to antigenreceptor ligation-induced activation of the phosphoinositide 3-kinase (PI-3-kinase) and Ras / MAPkinase pathways, events critical for lymphocyte proliferation. Such desensitisation reflects ES-62-primed recruitment of a number of negative regulators of these pathways, such as the phosphatases SHP-1 and Pac-1.

Rapid Translation System (RTS) is a cell-free protein production system employing an enhanced Escherichia coli lysate to perform coupled in vitro transcription-translation reactions. A continuous supply of energy substrates, nucleotides and amino acids combined with the removal of by-products guarantees a high yield of protein production. The gene to express is either cloned into a plasmid vector or introduced as a PCR product amenable to automation. The main property of this alternative system to cellular expression systems is its open design allowing direct manipulation of the reaction conditions and applications that are impossible or difficult in cell-based systems. RTS offers new promising possibilities in the postgenomic era.