Lecture 13

Prokaryotic Cell Shape and Cytoskeleton

Cytoskeletal Proteins in Prokaryotes

Prokaryotes maintain their shape through a cytoskeleton, which is a set of stable and dynamic protein filament structures capable of self-assembly and disassembly. These structures are essential for cell shape, mechanical strength, intracellular transport, cell movement, chromosome separation, cell division, and sub-cellular organization.

• FtsZ: A tubulin homolog and GTPase that forms a contractile ring at the site of cell division and recruits other proteins necessary for cytokinesis.

• MreB: An actin superfamily protein found encircling the cell, crucial for maintaining cell shape and facilitating cell division.

• Both proteins are essential for bacterial viability and morphogenesis.

Role of Cytoskeletal Proteins in Cell Shape

The arrangement and function of cytoskeletal proteins determine the morphology of bacterial cells, such as spherical, rod, and vibrioid shapes. FtsZ is involved in division, while MreB is associated with elongation and maintenance of rod shape. Crescentin is another protein that contributes to vibrioid shape.

• Spherical (e.g., S. aureus): FtsZ forms the division ring.

• Rod (e.g., B. subtilis, E. coli): MreB encircles the cell during elongation; FtsZ forms the division ring.

• Vibrioid (e.g., C. crescentus): MreB and crescentin contribute to elongation and curvature; FtsZ forms the division ring.

Changing Cell Shape: Genetic and Protein Influence

Mutations or deletions in cytoskeletal genes can dramatically alter cell shape. For example, loss of crescentin in C. crescentus results in straight rods instead of curved cells, demonstrating the importance of cytoskeletal proteins in maintaining morphology.

• Wild-type: Maintains characteristic shape.

• Mutant (creS::Tn5): Loses curvature, becomes straight.

• Complemented mutant: Regains curvature.

Prokaryotic Cell Compartments

The Periplasm

The periplasm is the space between the cytoplasmic membrane and an outside structure. In Gram-negative bacteria, it lies between the cytoplasmic and outer membranes; in Gram-positive bacteria, it is between the cytoplasmic membrane and peptidoglycan (though this is controversial).

• Functions: Protein folding systems, hydrolytic enzymes (phosphatases, proteases, endonucleases, amylases), transport, chemoreceptors, detoxifying enzymes (e.g., β-lactamases), and osmotic protection.

Bacterial Microcompartments

Bacterial microcompartments are specialized protein-bound compartments that optimize metabolic pathways involving toxic or volatile intermediates. These structures allow for the compartmentalization of specific biochemical reactions, protecting the cell from harmful intermediates.

• Metabolosomes: Microcompartments that contain enzymes for specific metabolic pathways.

• Gated pores: Regulate entry and exit of substrates and intermediates.

Carboxysomes: CO2 Fixation Shells

Carboxysomes are microcompartments found in some bacteria, such as Thiobacillus, that facilitate CO2 fixation. They contain enzymes like RuBisCO and carbonic anhydrase, which convert CO2 into organic compounds.

• Structure: Protein shell enclosing enzymes.

• Function: Enhances efficiency of CO2 fixation by concentrating substrates.

Biotechnological Applications of Microcompartments

Bacterial microcompartments are being engineered for biotechnological purposes, such as encapsulating desired proteins or drugs for targeted delivery and metabolic engineering.

• Protein encapsulation: Targeting peptides direct proteins into synthetic microcompartments.

• Drug delivery: Encapsulation of drugs like doxorubicin for controlled release.

Example: Metabolosome for 1,2-Propanediol Catabolism

The conversion of 1,2-propanediol to propionyl-CoA requires multiple enzymatic steps, often encoded by a large operon. The metabolosome compartmentalizes these reactions, preventing the accumulation of toxic intermediates.

• Operon: 21 genes encode enzymes and structural proteins for the pathway.

• Function: Efficient and safe catabolism of 1,2-propanediol.

Inclusions and Storage Structures

Food Storage Inclusions

Bacteria often contain dense aggregates of specific chemical compounds, serving as reservoirs for important nutrients. Poly-β-hydroxyalkanoate (PHB) is a common example, storing carbon and energy.

• Structure: Granules within the cytoplasm.

• Function: Storage of carbon and energy for later use.

Magnetosomes

Magnetosomes are specialized organelles in magnetotactic bacteria, containing magnetic particles that allow the cell to orient and swim along magnetic fields. The MamK protein is required for proper alignment of magnetosomes.

• Structure: Membrane-bound chains of magnetic crystals.

• Function: Navigation using Earth's magnetic field.

Other Specialized Compartments

Gas Vesicles

Gas vesicles are protein-bound structures found in some bacteria and archaea, such as cyanobacteria and Halobacterium. They provide buoyancy, allowing cells to adjust their position in the water column.

• Structure: Gas-filled, rigid, protein shells.

• Function: Floatation and positioning in aquatic environments.

Endospores

Endospores are highly resistant cell structures formed by some bacteria as a survival mechanism. They can withstand extreme conditions, including heat, radiation, chemicals, and desiccation.

• Structure: Multiple protective layers, including exosporium, spore coat, core wall, cortex, and DNA.

• Function: Long-term survival in harsh environments.

• Key compound: Calcium dipicolinate, which stabilizes the spore core.

Eukaryotic Organelles of Prokaryotic Origin

Mitochondria

Mitochondria are double-membraned organelles found in eukaryotes, believed to have originated from prokaryotic ancestors via endosymbiosis. They contain circular DNA and 70S ribosomes, similar to bacteria.

• Structure: Two membranes, many membrane proteins, circular DNA, 70S ribosomes.

• Function: Catabolism of nutrients via the TCA cycle and ATP synthesis.

Chloroplasts

Chloroplasts are organelles responsible for photosynthesis in plants and algae. Like mitochondria, they have a prokaryotic origin and contain circular DNA and 70S ribosomes.

• Structure: Two membranes, thylakoid membranes (site of photosynthesis), circular DNA, 70S ribosomes.

• Function: Conversion of light energy to ATP and reducing power.

Evidence for Endosymbiosis

Several lines of evidence support the endosymbiotic origin of mitochondria and chloroplasts:

• Both organelles contain bacterial chromosomes.

• Double membrane structure is similar to Gram-negative cell walls.

• Both divide by binary fission.

• Chloroplast photochemistry is similar to cyanobacteria.

• Ribosomes contain 16S rRNA, characteristic of prokaryotes.

Summary Table: Bacterial Compartments and Functions

Compartment	Main Function	Key Features
Periplasm	Protein folding, enzymatic degradation, transport, sensing, detoxification	Space between membranes
Microcompartments	Metabolic pathway optimization	Protein shell, gated pores
Carboxysome	CO2 fixation	Encloses RuBisCO, carbonic anhydrase
Inclusions	Food storage	Granules of PHB, glycogen, etc.
Magnetosome	Magnetic navigation	Membrane-bound magnetic crystals
Gas vesicle	Floatation	Gas-filled protein shell
Endospore	Survival	Multiple protective layers, calcium dipicolinate
Mitochondrion	ATP synthesis	Double membrane, circular DNA, 70S ribosomes
Chloroplast	Photosynthesis	Double membrane, thylakoid, circular DNA, 70S ribosomes

Additional info: Academic context was added to clarify the functions and significance of each compartment and structure, as well as to provide definitions and examples for student study.