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Recognize proteins in unfolded or denatured state. Recognize hydrophobic patches on the protein.
Can bind to proteins while being synthesized to keep in correct conformation.
Protein transit towards mitochondria/chloroplast requirement
Be in unfolded form to transit membrane.
A family of molecular chaperones (some found in cytosol).
Need ATP hydrolysis to release protein from chaperone.
Regulate traffic between nucleus and cytosol.
Factors to consider in nuclear transport
1) Nucleus surrounded by envelop composed of two concentric membranes (out and inner).
Both evolved from plasma membrane.
2) Traffic must go in 2 directions: Histones, gene regulatory proteins, DNA and RNA polymerase, and RNA processing protein must be able to get in.
tRNAs, processed mRNA, ribosomal subunits must get out.
3) Mitosis: Nuclear envelope disassembled and must reform.
Pores on the nucleus that regulate entrance/exit of molecules.
Signals for import of proteins to nucleus
4-8 positively charged amino acids
Recognized by nuclear import receptors. Proteins travel through pores in their fully folded state.
Signal is not cleaved after transport.
Supports inner membrane of nucleus and is meshwork of proteins called nuclear lamins.
Nuclear lamina roles
Gives shape and support and structural link between DNA and nuclear envelope.
Nuclear lamins become phosphorylated and ______
thought to trigger disassembly and nuclear envelope breaks into vesicles.
repolymerize on the surface of chromosomes and bind vesicles of envelope and fuse to reform envelope around chromosome. Then actively reimports all nuclear protein.
Differences from transit to ER
1) Signals are different.
2) Must cross double membrane of chloroplast and mitochondria to get into matrix/stroma. Usually one signal to cross both membrane and another signal to get into intermembrane space.
3. Mitochondria has an electrochemical gradient that helps initial penetration of membrane.
4. Needs ATP hydrolysis to achieve transit.
5. Transit occurs posttranslation.