Question

Post-translational modification

Protein targeting to ER & elsewhere; signal peptide & signal recognition particle. Why are some proteins targeted to the ER? Compare secreted proteins to integral membrane proteins.

Free vs. bound ribosomes

How is the control of gene expression different in prokaryotes & eukaryotes?

Answer 1

The fate of protein synthesized by cytosolic ribosomes depend on their amino acid sequnece, which contain sorting signals that direct their transport from the cytosol into the nucleus, the ER, mitochondria, plastids or peroxisomes. Protein without sorting signal remain in the cytosol as permanent residents. Within the cell, translocation of cytosolic proteins occur in three different ways -

1. ?Gated transport - protein movement between the cutosol and nucleus through the nuclear pore.
2. Transmembrane transport - Transport of protein across the membrane from the cytosol into the ER lumen, mitochondria, plastids or peroxisomes.
3. Vesicular transport - Transport of proteins from one compartment to another through transport vesicles. The transfer of soluble proteins from the ER to the golgi apparatus.

?Protein translocation may occur co-translationally or post translationally. proteins synthesized by membrane bound ribosomes are translocated co-trabslationally.The membrane free ribosomes synthesize all proteins that are translocated post-translationally.

Proteins synthesized by membrane bound ribosomes include soluble and membrane proteins that reside in the ER, resident proteins in the lumen of golgi complex and lysosomes, integral proteins in the membrane of these organelles and the plasma membrane and proteins that are secreted from the cell. Proteins destined to be secreted move through the secretory pathway -- RER ---> ER to golgi transport vesicles ----> Golgi cisternae ----> secretory or transport vesicles ----> cell surface