Inclusions are common in all cells. They are formed by the aggregation of substances that may be either organic or inorganic. The first bacterial inclusions were discovered in the late 1800s. Since then much has been learned about their structure and function. Inclusions can take the form of granules, crystals, or globules; some are amorphous.
Some inclusions lie free in the cytoplasm. Other inclusions are enclosed by a shell that is single-layered and may consist of proteins or of both proteins and phospholipids. Some inclusions are surrounded by invaginations of the plasma membrane. Many inclusions are used for storage (e.g., of carbon compounds, inorganic substances, and energy) or to reduce osmotic pressure by tying up molecules in particulate form.
The quantity of inclusions used for storage varies with the nutritional status of the cell. Some inclusions are so distinctive that they are increasingly being referred to as microcompartments. A brief description of several important inclusions follows.
Cells have a wide variety of storage inclusions. Many are formed when one nutrient is in ready supply but another nutrient is not. Some store end products of metabolic processes. In some cases, these end products are used by the microbe when it is in different environmental conditions. The most common storage inclusions are glycogen inclusions, polyhydroxyalkonate granules, sulfur globules, and polyphosphate granules.
Some storage inclusions are observed only in certain organisms, such as the cyanophycin granules in cyanobacteria. Carbon is often stored as polyhydroxyalkonate (PHA) granules.
Several types of PHA granules have been identified, but the most common contain poly-β-hydroxybutyrate (PHB). The structure of PHB inclusions has been well studied, and PHB granules are now known to be surrounded by a single-layered shell composed of proteins and a small number of phospholipids. Much of the interest in PHB and other PHA granules is due to their industrial use in making biodegradable plastics.
Polyphosphate granules and sulfur globules are inorganic inclusions observed in many organisms. Polyphosphate granules store the phosphate needed for synthesis of important cell constituents such as nucleic acids. In some cells, they act as an energy reserve, and polyphosphate also can serve as an energy source in some reactions, when the bond linking the final phosphate in the polyphosphate chain is hydrolyzed.
Sulfur globules are formed by bacteria that use reduced sulfur-containing compounds as a source of electrons during their energy-conserving metabolic processes. For example, some photosynthetic bacteria use hydrogen sulfide (rather than water) as an electron donor and accumulate the resulting sulfur either externally or internally.
Some bacterial inclusions are unique and serve functions other than simply storing substances for later use by the cell. These inclusions are called microcompartments. Microcompartments share several characteristics. They are relatively large polyhedrons formed by one or more different proteins. Enclosed within the protein shell are one or more enzymes. Microcompartments include the ethanolamine utilization (Eut) microcompartment, the propandiol utilization (Pdu) microcompartment, and carboxysomes. We focus here on carboxysomes as they are the best studied.
Carboxysomes are present in many cyanobacteria and other CO2-fixing bacteria. Their polyhedral coat is composed of about six different proteins and is about 100 nm in diameter. Associated with the shell is the enzyme carbonic anhydrase, which converts carbonic acid into CO2.
Recall that biological membranes allow the free diffusion of CO2. However, the carboxysome shell prevents CO2 from escaping so it can accumulate within. Enclosed within the polyhedron is the enzyme ribulose-1, 5-bisphosphate carboxylase/oxygenase (RubisCO).
RubisCO is the critical enzyme for CO2 fixation, the process of converting CO2 into sugar. Thus, the carboxysome serves as a site for CO2 fixation. As such, it is critical that carboxysomes be distributed to both daughter cells during cell division. A recent study has demonstrated that ParA, a cytoskeletal protein, helps ensure appropriate segregation of carboxysomes.
Inclusions can be used for functions other than storage or as microcompartments. Two of the most remarkable inclusions are gas vacuoles and magnetosomes. Both are involved in bacterial movement. The gas vacuole provides buoyancy to some aquatic bacteria, many of which are photosynthetic. Gas vacuoles are aggregates of enormous numbers of small, hollow, cylindrical structures called gas vesicles.
Gas vesicle walls are composed entirely of a single small protein. These protein subunits assemble to form a rigid cylinder that is impermeable to water but freely permeable to atmospheric gases. Cells with gas vacuoles can regulate their buoyancy to float at the depth necessary for proper light intensity, oxygen concentration, and nutrient levels.
They descend by simply collapsing vesicles and float upward when new ones are constructed. Aquatic magnetotactic bacteria use magnetosomes to orient themselves in Earth’s magnetic field. Magnetosomes are intracellular chains of magnetite (Fe3O4) or greigite (Fe3S4) particles. They are around 35 to 125 nm in diameter and enclosed within invaginations of the plasma membrane.
The invaginations have been shown to contain distinctive proteins that are not found in the rest of the plasma membrane. Each iron particle is a tiny magnet: The Northern Hemisphere bacteria use their magnetosome chain to determine northward and downward directions, and swim down to nutrient-rich sediments or locate the optimum depth in freshwater and marine habitats.
Magnetotactic bacteria in the Southern Hemisphere generally orient southward and downward, with the same result. For the cell to move properly within a magnetic field, magnetosomes must be arranged in a chain. A cytoskeletal protein called MamK is currently thought to be responsible for establishing a framework upon which the chain can form.