Bio Chem Topic #4 (Lectures 10-12)

Things that occur in ER

GPI anchoring, glycosylation, Disulfide bonds, Folding

GPI Anchoring Steps

Proteins destined for plasma membrane embedded into membrane via C term STA signal

GPI Transamidase cleaves soluble part of protein and links to GPI via amide bond in ER

N-Linked glycosylation

oligosaccharide linked to lipid carrier (dolichol)

transferred to asparagine residues in Asn-X-Ser/Thr

oligosaccharyltransferase (OST)

transfers the oligosaccharides to the asn in asn+o linked residue sequence in N linked glycosylation

glucosidases I and II

trim the three-ending glucose residues on the oligosaccharide part of calnexin/calreticulin(chaperone proteins) cycle to ensure proper folding

ER mannosidases

remove mannose residues

Disulfide bond formation in ER

occurs between cysteines mediated by oxidized PDI

Reduced PDI oxidation

oxidized by oxidized Ero1 protein

Protein degradation pathways

proteosomes (via ubiquitination pathway)

Lysosomes

M6P Tagged proteins

Delivered to lysosome via clathrin coat proteins

Peroxisomal Import

Folded proteins imported into peroxisome w/o losing targeting sequence

PST1

Peroxisomal targeting sequence (Ser-Lys-Leu) at C term recognized by Pex5 receptor

Pex14

Peroxisomal bound receptor that binds to pex5+protein complex and inserts protein into peroxisome

Pex5 Fate

After delivery, monoubiquitinated(by pex 2+10+12) and extracted from the membrane by AAA ATPase complex (pex 1+6)

Tim 23

Voltage gated channel that facilities translation of positively charged target sequences in mitochondria

Import of proteins into mitochondria

HSC70 cytosolic keeps protein unfolded during transport to mitochondria membrane

Binds with Tom

Then Tim

Matrix Hsc70 folds via ATP hydrolysis 

Pex12 deficiency

proteins can enter membrane of peroxisome but not matrix

Pex3 deficiency

proteins can not enter peroxisome matrix or membrane

Nuclear pore complex structure

Huge

Hydrophobic nucleoporin basket (phenylalanine and glycine)

NLS (nuclear localization signal)

rich in positive AA like lysine and arginine

Importin mediated nuclear import

Importin binds with cargo

Importin protein complex diffuse through nuclear pore

Ran-GTP binding in nucleus triggers cargo release

Importin Ran GTP complex exit through nuclear pore

Ran-GTP/GDP Cycle In Nucleus

In Cytoplasm Ran-GTP is converted to GDP by Ran GAP

In nucleus Ran GDP is converted to GTP by Ran GEF

Nuclear Export Signal

hydrophobic often rich in leucine

Exportin mediated nuclear export cycle

Triple complex formed exportin, protein and ran GTP

Complex diffuses through nuclear pore

Ran GAP converts GTP to GDP in cytoplasm triggering cargo release

Exportin diffuses back through nuclear pore with Ran GDP

Ran independent export

NXF1 and NXT1 complex binds to mRNA protein complexes and diffuse through nuclear pore visa FG nucleoporin interactions

Microtubules structure and function

ab tubulin. A tubulin-GTP and B Tublin-GDP

Track for kinesins and dyneins

Microfilaments structure and function

Actin

Tracks for myosins

Treadmilling of Actin

ATP actin at + end faster than ADP actin at - end

Actin Motility (treadmill)

Can move cells in some immune cells like neutrophils

Actomyosin cycle

ATP binds myosin leaves actin

ATP hydrolysis causes conformational change in myosin

myosin rebinds w/actin at different spot pi released

power stoke occurs when myosin returns to og conf sliding actin filament with it 

Regulation of myosin (prevents continuous muscle contraction)

tropomyosin and troponin complex blocks myosin binding sites on actin. With increase of ca2+ conc, these sites are exposed allow for myosin binding.