Signal transmission function

The potential of the combination of a signal processing function (active function) using NLO polymers with a signal transmission function (passive function) of transparent... 

Ras and its relatives are subjects of intensive investigations by biological, biochemical, biophysical, and medical studies. Within just one decade more than 17,000 articles (Medline, 1966-2000) deal with function and properties of this protein. Structural and functional data, based on Ras as a prototype, have provided insight into the basic principles of GTP-binding proteins, their activation, de-activation, and signal transmission. 

What is a synapse In the brain, the nerve cells or neurons are connected at special functional junctions called synapses, which depend on many proteins, including large complexes. They participate in basic functions with important roles in coordinating every characteristic of the nervous system, including physiology, emotions, learning, sleep, memory, and pain signal transmission. 

The regulatory GTPases function as switches that can exist in an active or inactive form. In the active form the GTPases can transmit signals to downstream components in the signaling chain. In the inactive form signal transmission in repressed. 

Fig. 5.16. Functional cycle of the heterotrimeric G-proteins. a) The G-proteins exist in the ground state as a heterotrimeric complex (G GDP) (Py)- b) The activated receptor binds to the inactive heterotrimeric complex of the G-protein and leads to dissociation of the bound GDP and the Pyeomplex. c) Binding of GTP to the empty G -subunit transforms the latter into the active G GTP state. G GTP interacts with an effector molecule in the sequence El and activates the latter for further signal transmission. The released Py-complex may also take part in signal conduction by binding to a corresponding effector molecule E2 and activating the latter for further signal conduction, d) Hydrolysis of the bound GTP terminates the signal transduction via the a-subunit.

The interaction of G GTP with the corresponding effector molecule leads to inactivation of the former and thus to initiation of the next step in the signal transmission chain. The Pycomplex released during activation can also perform a signal-mediating function (see 5.5.7). 

Originally, it was assumed that the Py-complex only played a passive role in the functional cycle of the G-proteins. It soon became apparent, however, that the Py-complex, in addition to binding to the a-subimit, also carries out other functions and interacts specifically with corresponding effector molecules (review article Neer, 1995). Tlie Py-complex must be assigned its own regulatory function, it takes part itself in the propagation and termination of signal transmission. 

The interaction of the Pycomplex with G-protein coupled receptor kinases (see 5.3.4, P-adrenergic receptor kinase, PARK) appears to be of special regulatory importance. The function of the Py-complex in this system is shown in Fig. 5.9. The Py-complex binds specifically to the PARK and translocates this to the cell membrane. The translocation of PARK is necessary to switch off and modulate signal transmission via adrenaline. 

Binding of phosducin to the Pycomplex leads to its translocation from the membrane into the cytosol. In this way, the number of Py-complexes available for the G-protein cycle is reduced and signal transmission is weakened. Interestingly, the phosdu-cin function is subject to regulation by phosphorylation. In the Ser-phosphorylated form, binding to the Py-complex is greatly weakened. 

Fig. 6.1. Function and formation of intracellular messenger substances in signaling pathways. Starting from the activated receptor, effector proteins next in seqnence are activated that create an intracellular signal in the form of diffusible messenger snbstances. The hydrophihc messenger substances diffuse to target proteins in the cytosol and activate these for signal transmission fnr-ther. Hydrophobic messenger substances, in contrast, remain in the cell membrane and diffuse at the level of the cell membrane to membrane-localized target proteins. PK protein kinase S substrate of the protein kinase.

It is easy to see that nonpolar, lipophilic molecules will easily cross membranes. But we know that polar metabolites and ions must also gain entrance to as well as exit from cells and organelles. This is accomplished through another type of membrane component - membrane proteins. These proteins may lie on the surface of the membrane (peripheral proteins) or be located either entirely within the nonpolar interior, or be partially exterior and interior, or completely span the bilayer (integral proteins). See Figure 2c. These proteins function as channels, carriers, receptors, or signal transmission devices. 

Reagents and indicators are immobilized, occluded or dissolved in supports which are formed by cross-linked polymers, plasticized polymers or organic and inorganic activated surfaces. The waveguide itself, the cladding of an optical fiber or any other optical element can be the support. However, it must obey two basic functions act as a liquid-solid or gas-solid interface and, if radiation crosses through it to allow the signal transmission, be an optically transparent material. 

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