Chapter 4: Archaeal Cell Structure

4.1 Archaea are Diverse, but Share Some Common Features

Archaea

• Best known for growth in:

  • Anaerobic

  • Hypersaline

  • Extreme pH

  • High-temperature habitats

• Highly distinct with respect to morphology, physiology, reproduction and ecology

Archaeal Shape, Arrangement and Size

• Common shapes are cocci and rods

• Other shapes can exist

  • Spirochete-like and mycelial forms have yet to be discovered

  • Unique branched and flat shapes have bene observed

• Sizes vary

  • Rodes

    • 1-2 microm W

    • 1-5 microm L

  • Cocci

    • 1-3microm Diameter

    • Smallest observed is .2microm

  • The largest so far is a multicellular form that can reach 30mm in length

4.2 Archaeal Cell Envelopes are Structurally Diverse

Archaeal Cell Envelopes

• Differ from bacterial CE in molecular makeup and organization

  • S-layer may be only component outside PM

  • Some lack CW

  • Slime layers are observed to mediate cell-cell interactions, but little is known abt composition and regulation

• Most common CE is an S-layer composed of many copies of a single protein

• Function

  • Protection

  • Cell Shape

  • Cell Membrane Stabilization

  • Molecular sieve

    • Regulates transport of materials to cell

• Archaeal S-Layer Structurer

  • Can be up to 70nm thick

  • Tethered to the PM

    • Resembles a protein canopy from the side

    • Geometric pattern from the top

Archaeal Membranes

• Composed of unique lipids

  • Hydrocarbons derived from isoprene units (5-C, branched molec)

  • Hydrocarbons attached to glycerol by ether linkages instead of ester linkages

  • Two Major Types

    • Glycerol diether lipids

      • Hydrocarbons (20C) attached to glycerol

      • C20 diethers make a bilayer

    • Diglycerol tetraether lipids

      • 2 hydrocarbons (40C) attached to 2 glycerol

      • Tetraethers are more rigid than diethers

      • C40 diethers make a monolayer

  • Also Found

    • Polar phospholipids

    • Sulfolipids

    • Glycolipids

• Some have a monolayer structure instead of a bilayer structure

Archaeal Extracellular Vesicles

• Composed of the PM and CW material

• Cargo includes cytoplasmic contents

  • Proteins

  • Nucleic Acids

• Important for intracellular gene transfer in thermophiles

  • Protects DNA from denaturing in high temperature

Archaeal Cells + Nutrient Uptake

• Similar to bacterial system

  • Passive and Facilitated Diffusion

  • Primary and Secondary Active Transport

  • Group translocation mechanism has been found in some

4.3 Archaeal vs Bacterial Cytoplasm

• Very Similar - lack of membrane-enclosed organelles

• May contain inclusion bodiea

Ribosomes

• Same size as bacterial

• Differ in composition

  • 70s constructred of a 50S and 30S subunit

  • rRNA similar size to bactera, but different nucleotide sequence

  • Protein composition also differs

    • Archaea more similar to eukarya

    • This different composition makes archaeal ribosomes unaffected by antibiotics that target the bacterial ribosomes

Nucleoid

• Revion in cytoplasm where supercoiled chromosome + nucleoid-associated proteins are aggregated

• Histones are associated w chromosome

• Irregulary shaped

• Usually not membrane bound

4.4 Archaeal External Structures

Pili

• Composed of pilin proteins that are made in cytoplasm + then anchor to a protein complex in the PM

• Two archaeal pili:

  1. Cannulae - hollow, tube-like structures on surface of thermophilic archaea

     1. Shown that daughter cells that arise from cell division remain connected to each other by cannulae

  2. Hami - resemble grappling hooks

     1. May function to attach cells to surfaces

     2. Seen in biofilm communities

Motility

• Flagella thinner than bacteria

• Filament is not hollow

• Rotation

  • Powered by ATP hydrolysis instead of proton motive force

  • Direction moves cell forward/backwards rather than running and tumbling