BIOCHEMISTRY - L. Stryer - 1984

VOLUME 3

PART V. MOLECULAR PHYSIOLOGY

CHAPTER 37. EXCITABLE MEMBRANES AND SENSORY SYSTEMS

37.20. Bacterial Chemoreceptors Detect Specific Molecules and Transmit Signals to Flagella

At the end of the 19th century, German botanist Wilhelm Pfeffer demonstrated that motile bacteria accumulate around the opening of a fine capillary containing an attractant, such as sugar (Fig. 37.38). Conversely, when the capillary contains a repellent (typically a substance harmful to bacteria or a metabolic waste product), the bacteria swim away from it. This directed movement of bacteria toward certain substances and away from others is known as chemotaxis. In the 1960s, Julius Adler began studying the molecular basis of bacterial chemotaxis. Biochemical, genetic, and structural studies conducted by him and many other researchers have elucidated numerous aspects of this process. Chemotaxis begins with the detection of chemical compounds by specific chemoreceptors on the cell surface. Information from these sensors is relayed to a transduction system, where a large number of stimuli are analyzed and integrated. Next, the sensory transduction system sends signals to the motors that drive the flagella. Depending on these signals, the bacterium either swims smoothly in a straight line or abruptly changes its direction of movement.

Class="center">Fig. 37.38. Bacterial chemotaxis. Bacteria move toward a capillary containing an attractant, such as glucose

About 20 different chemoreceptors have been identified in E. coli. Each of these Proteins is localized either in Cell/30.html">The Plasma Membrane or in the periplasmic space. A chemoreceptor consists of a recognition component and a signaling component. In chemoreceptors that mediate positive chemotaxis (attraction), the recognition component turns out to be a binding protein involved in The transport of that compound into The Cell. For example, the galactose-binding protein (a soluble protein localized in the periplasmic space) serves both as the recognition component for positive chemotaxis toward galactose and as part of the pump that actively transports galactose into the cell. The glucose chemoreceptor is also a component of the membrane-bound phosphotransferase system responsible for the active uptake of this sugar (Sec. 36.12). E. coli also possesses chemoreceptors for attractants such as Serine, Cysteine, Alanine, and Glycine. Although transport and chemotaxis are closely linked, chemotaxis is independent of transport processes. For instance, certain mutants unable to transport specific sugars or Amino Acids retain The ability to move toward them directionally. There are also various chemoreceptors associated with negative chemotaxis. Fatty acids, alcohols, hydrophobic amino acids, indole, H⁺ (pH < 6.5), OH^- (pH > 7.5), and sulfides repel bacteria by interacting with specific chemosensors.

Fig. 37.39. Electron micrograph of S. typhimurium. It is clearly visible that the flagella are bundled together

37.21. The Base of the Bacterial Flagellum Houses a Reversible Rotary "Motor"

Bacteria swim by means of the rotation of flagella protruding from the cell surface. These thin helical filaments are composed of 53-kDa subunits called flagellin. E. coli has about 6 flagella, each 10 µm long and 150 Å in diameter. Compared with eukaryotic flagella and cilia (Sec. 34.18), bacterial flagella are significantly smaller and simpler in Structure. A bacterial flagellum cannot actively undulate on its own because it lacks a contractile apparatus. Instead, it is driven by a "motor" located at the junction between the flagellum and the cell envelope. The isolation of flagella retaining their attached basal structures has made it possible to study these assemblies; they were found to consist of a filament, a hook, and a rod. In E. coli, four rings are mounted on the rod. The outer ring is attached to the outer membrane, while the inner ring is anchored to the plasma membrane of the cell envelope. The basal body (Fig. 37.40) anchors the flagellum to the cell envelope and sets it in motion. This "motor" is powered by the proton-motive force generated across the plasma membrane, rather than by the energy of ATP Hydrolysis. Indeed, the angular velocity of rotation is directly proportional to the proton-motive force. A puzzling property of the "motor" is that it can rotate both clockwise and counterclockwise.

Fig. 37.40. The basal body of the E. coli flagellum

Typically, an individual E. coli bacterium swims smoothly in a straight line for about one second. During this time, it covers a distance of roughly 30 µm, which is about 15 times its own length. The bacterium then tumbles and abruptly changes its direction of movement (Fig. 37.41). The angle of the turn typically averages about 60°. What determines whether a bacterium swims smoothly or tumbles? It turns out that when the flagella rotate counterclockwise, their helical filaments organize into a coherent bundle that drives the smooth forward movement of the cell. If, however, the flagella rotate clockwise, the entire bundle flies apart, each filament pulls in its own direction, and the bacterium begins to tumble.

Fig. 37.41. Projection of an E. coli swimming path obtained via an Electron microscope by automatically tracking bacterial movement in three dimensions. The dots are separated by 80 ms intervals

37.22. Bacteria Detect a Temporal Gradient Rather Than an Instantaneous Spatial Concentration Gradient

The regulation of tumbling frequency is central to chemotaxis. When a bacterium moves in the direction of an increasing attractant concentration, tumbling becomes less frequent. Conversely, when moving away from an attractant, the bacterium tumbles more often. Repellents have the opposite effect on tumbling frequency. As a result, when moving toward an attractant or away from a repellent, a bacterium swims in a straight line for a longer period than when moving in the opposite direction. Tumbling thus helps select the correct direction.

Fig. 37.42. Bacterial tumbling is caused by an abrupt 180° reversal in "motor" rotation, resulting in the flagellar bundle flying apart in various directions

Does a bacterium compare the concentrations of an attractant at the two ends of its cell, or does it compare concentrations at two distinct points in time? In other words, is its sensory mechanism spatial or temporal? Daniel Koshland and Robert Macnab found the answer to this important question by performing a very simple and ingenious experiment. A suspension of bacteria in an attractant-free medium was rapidly mixed with a solution containing an attractant, and the tumbling frequency of the bacteria was measured. Strikingly, just one second after mixing, this frequency decreased. The bacteria swam relatively long distances in straight lines, even though no spatial concentration gradient existed in the rapidly stirred solution. Consequently, bacteria perceive changes in concentration over time—that is, they possess a temporal sensory mechanism. In other words, a bacterium detects an attractant gradient in space not by comparing its concentration at the front and rear ends of its cell, but by comparing successive concentration perceptions over time as it moves. Essentially, bacterial chemotaxis is a partially biased random walk. Directionality arises from the Selection of random directions of movement.

37.23. Information Transmission in Bacterial Chemotaxis Is Mediated by Methylated Proteins

The bacterial response to a given attractant concentration gradient is not strictly fixed. The suppression of tumbling caused by an increase in

concentration of the attractant is transient. Following a certain lag period, the tumbling frequency returns to its initial level. Essentially, in the continuous presence of the attractant, the bacterium loses its sensitivity to it. Thanks to this desensitization, known as adaptation, bacteria are able to perceive gradients across a wide range of attractant concentrations. As noted earlier, rod photoreceptors in the retina exhibit a similar property in response to light (Fig. 37.30).

Genetic studies of bacterial chemotaxis have shown that information from at least 12 chemoreceptors is transmitted via 3 protein products of the tsr, tar, and trg genes (Fig. 37.43). For example, signals from ribose and galactose receptors are relayed to the trg protein. These three proteins undergo reversible methylation, which is why they are called methyl-accepting chemotaxis proteins (MCPs). Several glutamate side chains in each of these proteins are methylated by a methyltransferase that uses S-adenosylmethionine as an activated methyl group donor (Fig. 37.44). These covalent modifications can be reversed by a specific esterase. The methylation level of MCPs increases upon exposure to an attractant and decreases in response to a repellent. Apparently, adaptation is mediated by the degree of methylation of these proteins.

Fig. 37.43. Information transmission pathway in bacterial chemotaxis. The bottom part of the diagram shows identified mutant types. MCP - methyl-accepting chemotaxis protein

Fig. 37.44. Reversible methylation of methyl-accepting chemotaxis proteins

Information from these three methyl-accepting proteins is then passed on to the che Gene products. Exactly how these molecules determine the direction of flagellar rotation remains unknown. However, it is now entirely clear that bacteria possess a primitive sensory system formed by the products of roughly 30 genes, which processes and integrates information regarding nutrients and toxins in the bacterium's environment. Ultimately, the fundamental question for the bacterium is: "to tumble or not to tumble?"