The periplasmic receptor proteins

Signal Transduction Chemotaxis is the name given to the chemotactic mechanism exhibited by the gram negative bacteria. It involves a signalling cascade in which the presence of external chemotactic molecules is sensed by the bacterium and this presence is converted into a signal to stimulate movement. The movement which results is either towards the chemotactic molecule or away from it, and the chemotactant may be at a very small concentration compared to the level of intracellular components which it ultimately mobilises. The effect is to cause various cellular components to increase in their concentrations in a defined and highly regulated order, whereby one molecule in the cascade elicits the activation of many downstream molecules.

Gram negative bacteria cells have a double membrane arrangement. The outer membrane is highly permeable to many compounds, ranging from water to sugars and dipeptides, the inner membrane is very impermeable. This arrangement essentially provides a filtering system, and is the basis of the transduction mechanism. Chemotactic molecules build up in the periplasmic space, and some are recruited by periplasmic receptor proteins to permit them to interact with the downstream MCP’s.

MCP’s come in four distinct types, Type I / II / III / IV. Types I and II MCP’s respond to amino acids, whereby Type I responds to Serine, and Type II responds to Aspartate. Type III responds to sugars and Type IV to dipeptides. The interaction of chemotactants, which the bacterium encounters in its external environment, with the MCP’s is facilitated by a number of periplasmic located proteins, called the periplasmic receptor proteins.

These proteins mediate the interaction of chemotactants with the Type III and Type IV MCP’s. Hence, they are concerned with the binding of sugars and dipeptides, which have entered the periplasmic space from the bacterium’s external environment and their recruitment to the inner bacterial membrane. Other chemotactants, such as amino acids do not need transducing periplasmic receptor proteins to allow them to interact with the MCP’s, but do so via a direct intermolecular interaction.

The chemotactants to which the bacterium responds have freely entered the periplasmic space from the external environment via simple diffusion down their respective concentration gradients. Their passage across the outer bacterial membrane is unhindered, but many accumulate in the periplasmic space, being excluded from further entry by the inner bacterial membrane. Their presence is responded to by the MCP’s, which are activated by the chemotactants either directly, in the case of the Types I and II MCP’s, or via the transducing periplasmic receptor proteins in the case of the Types III and IV MCP’s.

The interaction with the MCP’s serves to alter an intracellular signalling cascade in the bacterium, the result of which is that chemoattractants elicit anti-clockwise flagellar rotation, and chemorepellents elicit clockwise flagellar rotation. The periplasmic receptor proteins communicate the presence of chemotactic molecules in the external environment to the Methyl Accepting Chemotaxis proteins (MCP’s). The MCP’s then transduce these signals into an intracellular signalling cascade which stimulates the appropriate mode of flagellar rotation.

The MCP’s are a family of closely related transmembrane receptor proteins. They have a homodimeric structure which has been revealed by x-ray diffraction techniques. Their three dimensional structure suggests that they are embedded in the inner membrane of the bacterium, with a periplasmic – located binding domain and intracellular coiled – coil domains. The periplasmic domains of the MCP’s serve to bind chemotactic molecules either directly or via the mediation of a periplasmic receptor protein. The intracellular coiled – coil domains contain the methylation sites which are vital in the process of adaptation. There are four methylation sites on each of the two polypeptide chains which constitute the homodimeric MCP. Additionally, each of the two polypeptide chains has a signalling domain, which is vital in the transduction process.

The transduction of the presence of the chemotactic molecule into an intracellular effect is a key stage in the process. Through gene studies, it has become clear that there are four cytoplasmic proteins involved in the signalling cascade downstream of the MCP’s. These proteins are CheA, CheW, CheY and CheZ. They effectively couple the MCP’s to the flagellar motor, and do this by using a phosphorylation relay system.

Only cells which are motile, that is, those cells which are capable of autonomous movement are able to carry out chemotaxis. The process of chemotaxis is limited to select types of specialised cells which possess the ability to detect changes …

Write an essay in up to 2000 words on the following topics, describing the procedures that can be used to isolate proteins from a cell and the methods involved in checking the purity. Using diagrams to help, you should explain …

Cimetidine is the most prototypical drug ever to be tested and shown to have exciting antagonistic properties with histamine H2-receptor. After GlaxoSmithKline’s development in about mid 1960s, a number of histamine H2 receptor antagonists have been developed. H2 receptor antagonists …

This was planned by discussing the actual food products we commonly take in a day. There are few diary products that are commonly known to contain a lot of protein, necessary carbohydrates for each meal (bread, pasta and cereal) as …

David from Healtheappointments:

Hi there, would you like to get such a paper? How about receiving a customized one? Check it out