Cell Bio Lecture #30 notes Chapter12 p.583-594

SRP binds ribosome+signal peptide, pauses translation takes ribosome+mRNA+ nascent protein to rough ER, translation resumes, protein fed across membrane.

Proteins cross mitochondrion., chloroplast posttranslationally, ER cotranslationally.(yeasts, bacterial membrane can do posttranslationally). ER pore opens, closes as complex is bound.

Single-pass transmembrane proteins: Stop-transfer peptide anchors protein in membrane. N-terminal start-transfer cleaved.

Or, start-transfer can be internal, and span membrane with N end in lumen, C in cytosol or reversed.

SRP (signal-recognition particle) scans unfolded polypeptide from N-terminus for hydrophobic stretches. 1^st hydrophobic = "start" 2^nd = "stop", etc.

Disulfide bonds not an option in cytosol (reducing environment).BiP (Binding protein) chaperone, keeps unfolded, drags proteins into ER lumen.

Glycosylphosphatidylinositol added to proteins destined for lumen of ER and cell surface (topological equivalence). Signal contained in hydrophobic C-terminus. Attach to exterior of plasma membrane by GPI terminus (many surface proteins done this way).

ER membrane makes nearly all lipids for cell membranes. Phosphatidylcholine, phosphatidyl- ethanolamine, phosphatidlyserine, phosphatidylinositol all done same way on cytosolic half, then, phospholipid translocators of ER for phosphatidyl choline in particular. So, asymmetric layers.

Phospholipid exchange proteins randomize phospholipids among all membranes; move lipids to mitochondrion, chloroplast. ER, Golgi, lysosomes, endosomes, plasma membrane all communicate.

Net movement = from high concentration membrane to low. So, phosphatidylcholine from ER to mitochondrion.

Polarities established in ER.