BIMM 120_After Midterm #2 Material_Saier Milton

Sec and Tat pathways:

-Sec pathway: Found in all living organisms. Moves unfolded proteins from cytoplasm to inner membrane via SecYEG complex, with SecA recruiting the complex using ATP/GTP while PMF stimulates translocation of unfolded proteins.

-Tat pathway: Found in archaea, bacteria, and chloroplasts. Transports fully folded proteins using the PMF, where TatA forms the pore, TatB helps transmit signal, and TatC recognizes signal peptide and energize system

ABC transporters:

-Found in prokaryotes and eukaryotes. Use ATP to move molecules from inside to outside the cell in 1 step and form transmembrane transporters with TMS. 

-ABC1: has 6 TMS and transports simple ions, small molecules, and proteins.

-ABC2: has 6 TMS, transports simple ions and complex macromolecules like LPS, and did not evolve from ABC1.

-ABC3: has 4 or 8 TMS, transports peptides and proteins, and evolved later.

Lol system:

Found in gram-negative bacteria to transport lipoproteins from IM to periplasm via LolA, a periplasmic chaperone, to LolB, an OM receptor that inserts them into the OM. Homologous but perform different functions.

Main Terminal Branch (MTB):

-Found in Gram-negative bacteria, spans both membranes, and exports proteins out of the cell.

-PulE: an ATP-binding ATPase/kinase with a zinc finger motif 

-PulL: required for PulE membrane association.

-PulE&L form Secretion: Transperiplasmic complex that recognizes substrates, energizes transport, and opens pore.

Fla/Path System:

uses ATP to inject proteins into cells, spans both membrane, and is homologous to T3SS and flagella.

F-type ATPase:

F-type ATPase: Rotary motor homologous to the bacterial flagellar system.

Conj/Vir System:

Type IV Pili secretion system (T4P SS) that uses ATP to mediate conjugation (DNA transfer) and virulence, and spans the IM and OM.

Type I–IV systems:

Distinct machines in Gram-negative bacteria that move proteins (or DNA) across one or both membranes by different mechanisms.

Type I Secretion System (ABC System):

Moves molecules from the inside the cell to outside the bacterium in 1 step using high substrate specificity, ATP-binding cassette transporter (hydrolyze ATP), MFP, and outer membrane protein to form a continuous tunnel.

Type II Secretion System (Sec/Tat + Secretin):

2-step system where proteins first cross the inner membrane via Sec for unfolded using ATP/GTP or Tat for folded using PMF, then cross the outer membrane through a Secretin or MTB pore.

Type III Secretion System (Fla/Path or Injectisome):

Uses ATP via YscN ATPase (hydrolysis) to transport cytoplasmic proteins to both bacterial membranes into the host cell, where YopB/YopD form a pore spanning the host membrane. homologous to flagella.

Type IV Secretion System (Conj/Vir):

Uses ATP to transfer plasmid DNA during conjugation or inject toxins during virulence, via multiple Vir proteins spanning the IM, periplasm, and OM. Example: VirB system of Agrobacterium causes cancerous plant growth via DNA transfer.

Protein Secretion and Membrane Insertion Systems:

-Holin proteins: Form oligomeric membrane pores to export autolytic enzymes that degrade peptidoglycan to cause cell death.

-Fimbrial Usher Protein (FUP) System: Export pilin subunits from inner membrane to lipopolysaccharide-containing outer membranes via the Sec system, chaperone proteins to prevent self assembly, and usher proteins to release.

Autotransporters: 

-AT-1: single proteins with signal peptide, passenger & beta domain that are secreted via Sec system, where the beta domain forms an outer membrane pore to release passengers.

-AT-2: contain transport & passenger domain linked by a coiled-coil linker, with fewer beta strands than AT-1.

Two-Partner Secretions TPS System:

composed of substrate and transport proteins that secrete large proteins with hemolytic or adhesive activities related to bacterial virulence.

OmpIP Family:

In Gram-negative bacteria, outer membrane proteins (OMPs) move to the periplasm via Sec and inserted into the outer membrane by Omp85, a universal component required for LPS and phospholipid assembly.