Signal molecules, peptides

The Src-homology 2 (SH2) domain is a protein domain of roughly 100 amino acids found in many signaling molecules. It binds to phosphorylated tyrosines, in particular peptide sequences on activated receptor tyrosine kinases or docking proteins. By recognizing specific phosphorylated tyrosines, these small domains serve as modules that enable the proteins that contain them to bind to activated receptor tyrosine kinases or other intracellular signaling proteins that have been transiently phosphorylated on tyrosines. 

Neuropeptide S (NPS) is a recently discovered bioactive peptide that has emerged as a new signaling molecule in the complex circuitry that modulates sleep-wakefulness and anxiety-like behavior. The peptide precursor is expressed most prominently in a novel nucleus located in the perilocus coeruleus, a brain structure with well-defined functions in arousal, stress, and anxiety. NPS was also found to induce anxiolytic-like behavior in a battery of four different tests of innate responses to stress. Infusion of NPS potently increases wakefulness and suppresses non-REM (NREM) and REM sleep (Xu et al, 2004). NPS binds to a G-protein-coupled receptor, the NPS receptor, with nanomolar affinity activation of the receptor mobilizes intracellular calcium. The NPS receptor is expressed throughout the brain, particularly in regions relevant to the modulation of sleep and waking, in the tuberomammillary region, lateral hypothalamus, and medial thalamic nuclei. 

The formylated peptide fMet-Leu-Phe is probably the most commonly-used activator of neutrophils in vitro. It is used as a model agonist to study receptor-mediated processes, generating intracellular signalling molecules that then activate cell functions. This compound can, depending upon the concentration used, activate many varied functions, such as chemotaxis, aggregation, reactive oxidant production, cytoskeletal changes and (particularly in combination with cytochalasin B) degranulation. 

In terms of their chemical structures, signalling molecules fall into five main categories (i) peptides, (ii) steroids, (iii) amino acids and their derivatives, (iv) fatty acid derivatives, and (v) nucleotides. 

In contrast to Gram-negatives, many Gram-positive bacteria employ post-translationally modified peptides processed from larger precursors as QS signal molecules. In Staphylococcus aureus, for example, a family of peptide (7-9 amino acid residues) thiolactones which vary in the primary amino acid sequence but contain a conserved cysteine at position 5 control the expression of cell wall colonization factors and exotoxins [24-26]. 

Certain proteins, peptides, steroids, and other small organic molecules, serve as cell messengers or mediators of signals. Hydrophilic mediators activate receptors at the surface of the target cell. Acetylcholine is a common example of such signaling molecules. It becomes bound on the exterior surface at the acetylcholine receptor of the nervous system, opening the channel. 

BIOACTIVE PEPTIDES AS SIGNAL MOLECULES IN PLANT DEFENSE, GROWTH, AND DEVELOPMENT... 

After an additional 10 years of research it is still not entirely clear what the signaling molecule is. Van Bel and Gaupels (2004) recently reviewed the possible signaling molecules that could induce SAR. The list includes jasmonic acid, lipid-derived molecules, reactive oxygen species (see Chapter 2, Section 1.9), oligosaccharides, mRNA molecules, calcium, and various peptides. 

Decho, A.W., Browne, K.A., and Zimmer-Faust, R.K., Chemical cues why basic peptides are signal molecules in marine environments, Limnol. Oceanogr., 43, 1410, 1998. 

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