* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Protein folding and movement in the bacterial cell The action of
Theories of general anaesthetic action wikipedia , lookup
Cell nucleus wikipedia , lookup
SNARE (protein) wikipedia , lookup
Cell membrane wikipedia , lookup
Magnesium transporter wikipedia , lookup
Cytoplasmic streaming wikipedia , lookup
Protein phosphorylation wikipedia , lookup
Bacterial microcompartment wikipedia , lookup
Protein structure prediction wikipedia , lookup
G protein–coupled receptor wikipedia , lookup
Nuclear magnetic resonance spectroscopy of proteins wikipedia , lookup
Protein moonlighting wikipedia , lookup
Protein domain wikipedia , lookup
Endomembrane system wikipedia , lookup
Protein folding wikipedia , lookup
Protein–protein interaction wikipedia , lookup
Signal transduction wikipedia , lookup
List of types of proteins wikipedia , lookup
Type three secretion system wikipedia , lookup
Trimeric autotransporter adhesin wikipedia , lookup
Intrinsically disordered proteins wikipedia , lookup
Protein folding and movement in the bacterial cell • All protein synthesis occurs in cytoplasm • Generally, product of translation is unfolded polypeptide, which must fold into proper 3 dimensional structure in order to function ! Polypeptide folding often will start before translation is finished, with " helices & # strands (Fig. 3.15) forming spontaneously !Tertiary/Quaternary (3°/4°) protein folding can occur spontaneously but frequently is aided by molecular chaperones • At least 20% of all polypeptides made ultimately are localized outside of the cytoplasm Localization of proteins to different cellular compartments After synthesis in cytoplasm, proteins destined for different cellular compartments are targeted there in different ways: • “Secreted proteins” that leave the cytoplasm before 3°/4° folding occurs, using a targeting signal that is removed during export • Proteins that are inserted into the cytoplasmic membrane (where they will fold) • Folded proteins that are exported with co-factors The action of chaperones Chaperones often hydrolyze ATP during 3°/4° folding of cytoplasmic proteins Unfolded or Chaperones can help protein attain its structure, but are not part of the final structure Hsp70 Hsp60 Fig. 7.32 Targeting signals for protein export across cytoplasmic membrane Proteins destined to cross the cytoplasmic membrane for final localization outside the cell (or in the periplasm/outer membrane of Gram neg. bacteria) generally have an Nterminal sequence that directs polypeptide to machinery that carries out the localization. One class of these targeting signals are used in both proks and euks to direct precursor proteins for secretion: signal sequences (SS or signal peptides or leader sequences). These SS are cleaved from precursor during localization process. General features of Signal Sequences • • • • N-terminal domain, 15-25 aa’s (longer in G+) First, region of 3-8 residues with 1-3 +aa Next, hydrophobic core: 7-15 hydrophobic aa’s SS cleavage site C-terminal to hydrophobic core (after Gly/Ala; sometimes more “polar”) Localization of precursor polypeptides depends on SS’s and their folding state • Cytoplasmic proteins stay in cytoplasm because they lack SS $They also fold quickly… • SS target polypeptides to export (or secretion/Sec) machinery • Export machinery (Sec apparatus) depends on unfolded state of polypeptide for localization $SS can antagonize folding of protein… See Fig. 3.12 for characteristics of aa side chains Bacterial secretion (Sec) apparatus and post-translational export Step 1: SecA + SS-precursor; SecB association Step 2: SecA cleaves ATP; SecEYG pore for translocation Essential: SecA, SecEY, Lep Helpful: SecB, SecG, SecDF Step 3: repeat ATP-driven SecA cycle feeding 20-30 aa segments through Sec; PMF-driven SecEYG cycle; SS-cleavage by Lep occurs early Secretion during translation (Co-translational export) SRP (signal recognition particle) can associate with some bacterial proteins like cytoplasmic membrane proteins during synthesis. SRP binds to receptor near Sec and facilitates Sec export into membrane Tat Secretion of folded proteins Secretion during translation (Co-translational) SRP is RNA/protein complex, largely targets proteins with very hydrophobic sequences (transmembrane domains) to Sec in Bacteria. For the membrane proteins, no SS cleavage. Bacterial SRP has similar structure but somewhat different function to euk SRP, which hasprimary role targeting SScontaining precursors to SEC. Fig. 7.33 Comparing the timing and apparatuses of export • • • Bacteria: lots of post and co-translational Sec; distinct SecA component; Bacteria with other more dedicated posttranslational export machineries such as Tat Archaea: No SecA but remaining apparatus very similar to Bacterial Sec; also has Tat export Eukaryotes: mostly co-translational SEC export; SEC in ER; no SecA component (Tat apparatus in chloroplasts) Alternative apparatus used in export of folded bacterial proteins: TatABC Tat substrates with distinct secretion signals (Twin Arg Tag); signal binds TatBC, then exported via TatABC, using PMF