chapter 9- multiprotein systems and proteomes

how are some cellular structural components not directly encoded by genes

non-protein components of bacterial cells, instead of being directly encoded in the genome, are synthesized by proteins

several protein enzymes come together in biosynthetic pathways to make the non-protein components

general characteristics of lipopolysaccharide

covers the outer surface of the outer membrane of gram-negative bacteria- some species produce LOS instead, which is shorter

contains lipid A, core, and O-antigen

lipid A portion anchors the large LPS molecule in the outer surface of the gram-negative outer membrane

core and O-antigen portions of the molecule extend out from the bacterial cell into the surrounding environment

lipid a portion of LPS

highly conserved between the various gram-negative bacterial species

9 genes are involved in the synthesis of lipid A- many present in clusters (operons and synteny maintained)

LPS core characteristics

oligosaccharide made up of 10-15 sugars connected directly to the lipid A part of the lipopolysaccharide

core itself is made up of the inner core, the outer core, and the kdo portion- which are made using enzymes encoded in three separate operons

O-antigen characteristics

repetitive glycan polymer of 1-8 sugars attached to the LPS core

structure, length, and characteristics varies between bacterial species and strains- variation in genes involved in biosynthesis

the variability of the o-antigen allows it to be used as the basis of serotyping in some species

how is LPS assembled and translocated in the cell

Components of the LPS are made in the cytoplasm, but their final place is on the outer surface of the bacteria; they require translocation across the membrane

Lipid A is embedded in the inner membrane (periplasmic side)

Then core and O-antigen are added

Finally, the complete LPS is translocated from the periplasmic leaflet of the inner membrane to the outer leaflet of the outer membrane

how is peptidoglycan in gram negative bacteria made

the precursors of the peptidoglycan cell wall are made in the cytoplasm —> periplasm for assembly into NAG and NAM

the cell wall also contains both L -and D- amino acids

many of the peptidoglycan biosynthesis genes are present within a cluster of genes alongside genes involved in cell division- coordinated processes

phospholipid characteristics

cellular membranes are mostly made up of phospholipids with their hydrophilic polar heads and hydrophobic non-polar tails

gram-negative bacteria have a second cellular membrane- the outer membrane: outer leaflet of the outer membrane contains both phospholipids and LPS

capsules characteristics

enables the bacteria to avoid being killed by the immune response

made up of extracellular polysaccharides that are anchored to the cell membrane through the lipid component of glycolipids

present outside of the outer surface of the gram-negative outer membrane, outside of the LPS, or beyond the thick peptidoglycan of the gram-positive bacteria

genes for the biosynthesis tend to be present in clusters

differences in capsule types are often regflected in differences in serogroups, which are used to differentiate bacterial strains phenotypically

proteins make up what bacterial cell structures

pili- long thin structures that protrude from the surface of the bacterial cell, made up of protein subunits

fimbriae- similar to pili, in that they are long thin protein structures extending from the bacterial cell (but shorter than pili and more present)

flagella- involved in motility, achieved through rotation of the flagella

what is the sec pathway

moves infolded proteins across the membrane and inserts them into the cytoplasmic membrane

a signal sequence present at the N-terminus of the protein

a signal recognition particle (SRP) binds signal sequence and delivers the protein to the Sec translocase

Tat pathway

twin arginine pathway

transports proteins destined for the periplasm and outside the cell

bacterial secretion systems move proteins across the bacterial membranes

proteins are secreted out of the cell- secretion systems

some secretion systems take secreted proteins across one membrane, two membranes, or even three membranes, when the proteins are secreted directly across a eukaryotic membrane and into the host cell

some secretion systems have evolved from other bacterial systems- pili, flagella, and bacteriophage

T 1 SS secretion system

takes proteins across both membranes in one step

T2SS secretion system

take a protein from the periplasm out of the cell

proteins cross the inner membrane using the Sec or Tat secretion systems

T3SS secretion system

inject proteins directly into host cells, delivering bacterial proteins directly to their targets

at the end of the needle is the translocon, which is able to sense contact with the host cell

translocon assembles on contact with the host cell and forms a pore in the host cell membrane

proteins secreted by T3SSs are called effectors and have a secretion signal for type 3 secretion within their N-terminus

prevalent specifically. ingram-negative pathogens and symbionts (operons with T3SS genes are typically gound in pathogenicity islands)

T4SS secretion system

includes conjugation systems and DNA uptake systems

mediated transfer of DNA between bacterial cells

T5SS secretion system

a T5SS protein mediates translocation into the outer membrane

First brough into the periplasm by the sec pathway

T6SS secretion system

the T6SS transports proteins into other cells

contractile nanomachine with a puncturing structure at the tip

gram-positive secretion systems can aid protein transport across the thick peptidoglycan layer

1-6 —> gram negative bacteria

gram positive bacteria have the sec and tat pathway to translocate proteins across the membrane

despite these systems, it is often not enough for a protein to escape the cell, in part this is due to the thick peptidoglycan layer

T7SS secretion system

transport proteins through both the membrane and the peptidoglycan layer, refucing the reliance on passive diffusion to secrete the protein out of the cell

what are efflux pumps

multiprotein systems that remove substances from the periplasm and/or cytoplasm - important role in antibiotic resistance

there are five fams of efflux pumps

powered by ATP or by the proton movtive force (PMF)

protons that are not used in the bacterial cell are exported to the surface and form an electrochemical gradient across the membrane that can be harnessed as en energy sourve for efflux pump transport