Chapter 8Lecture Outline

Transcription, Translation, and Bioinformatics

Transcription, translation

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8.5Secretion: Protein Traffic Control

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Protein Traffic Many bacterial proteins reside

in cytoplasm Others targeted to other sites

Plasma membrane Periplasm Gram outer membrane Secreted outside of bacterium

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Protein Secretion

Movement of proteins out of cytoplasm To the periplasm

Usually secreted unfolded Require a second set of proteins for correct folding

(chaperones) To the outer membrane Across both membranes to the surrounding

environment Special export systems are required Move hydrophilic proteins through hydrophobic

membrane barriers

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Protein Export to the Cell Membrane Proteins for cell membrane (e.g.cytochromes) are

tagged with very hydrophobic N-terminal signal sequences

Contain hydrophobic transmembrane spanning regions Signal recognition particle (SRP) binds to nascent N-

terminus and halt further translation Nascent protein with SRP and ribosome is delivered to

the membrane bound protein FtsY Two outcomes

Co-translational insertion into cell membrane Complete synthesis and then delivery to SecYEG translocon for

insertion

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SRP and Cotranslational Transport

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Protein SecretionAnimation: Protein Export

Click box to launch animation

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Examples are superoxide dismutase, maltose binding protein

Sec-dependent general pathway Protein completely translated in

the cytoplasm Protein captured unfolded by

SecB and delivered to SecA and the membrane-bound SecYEG

Protein is pushed by SecA through SecYEG in multiple steps

LepB cleaves signal sequence Periplasmic chaperones fold the

protein

Protein Export to Periplasm

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Examples are toxins Type I-VI secretion systems Type I are ABC (ATP Binding

cassette) transporters All have three components

ABC protein at the inner membrane

Periplasmic protein lashed to the inner membrane

Outer membrane channel Proteins secreted through Type I

never contact the periplasm Example: Hly (E. coli hemolysin)

Protein Secretion to the Outer Surroundings

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Bioinformatics: Genome Analysis Genome provides raw DNA sequence

What sequences encode proteins? Open reading frames (ORFs)

What sequences control expression? Promoters

What proteins perform specific functions?

Biochemical analysis of protein function

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Evolutionary Relationships Sequence reflects ancestry Homologs

Orthologs Genes duplicated via appearance of new

species Identical function in different organisms

Paralogs Genes duplicated within a species

Perform slightly different tasks in cell Can develop new capabilities