Rough Endoplasmic Reticulum

Lecture 19 BSCI 420/421 Oct 16, 2002

“If you want the rainbow, you gotta put up with the rain”-Dolly Parton

Fluorescent Micrographs of ER

TEMs of RER

F37FreeAndMembrane-BoundR-somes

The signal hypothesis: That an ER signal sequence in the nacentPolypeptide chain directs

Evidence for the signal hypothesis:

1. Without signal sequences, proteins are not imported into RER.(Protease protection assay)

2. When signal sequences are added to a gene coding for a normally cytosolic protein, the polypeptide it makes is incorporated into RER.

3. A different sized (smaller) polypeptide is made in RER than when the same mRNA is translated on free polysomes in anin vitro system (reticulocyte lysate wo RER).

The ER Signal is a sequence of about 10 hydrophobic AAsThat becomes anchored in the membrane by hydrophylic AAsOn either side. E.g.:Ser-leu-leu-leu-val-gly-Ile-leu-phe-trp-ala-thr near the NH2 terminal

The signal sequence initially binds to a RNP particle called theSignal Recognition Particle (SRP).

How the SRP directs ribosomes to the RER

Evidence for an aqueous pore thru which the polypeptidePasses into the ER

A soluble protein released into the ER lumen to be secreted

A single-pass membrane protein

Hydrophobicity plot of rhodopsin, a 7-pass transmembrane protein