Protein Production and Sorting

Organelle-specific uptake-targeting sequences - part of primary structure of protein

Act as address label - allow acesss into target organelles for sorting

Translocation at ER membrane

Vesicular transport links ER and cis-golgi

Maturation across medial golgi - linked w/ vesicles

Sorting of proteins at trans-golgi

• Secretory pathway begins w/ protein synthesis on ER membrane
• Protein produced by mRNA has ER signal sequence
• SRP recognises sequence and binds
• SRP binds to SRP recpetor on ER membrane 
• Ribosome binds
• SRP transfers ribosome/protein to translocon membrane protein
• SRP recycled
• Protein translocated to ER lumen as its translated
• Signal sequence cleaved off protein
• Ribsome dissociates
• Protein mods in ER

Membrane proteins - diff orientations/transvering numbers

More times transversed - more stable (embedded)

Stop-transfer anchor sequence:

• hydrophobic amino acids - interact w/ phospholipids = insertion into membrane
• Internal signal sequence recognised by SRP
• Positioning of signal sequence in primary structure = protein orientation affected

Newly synthesised ER proteins modded by folding w/ disulphide bonds, glycosylation, proteolytic cleavage, multimer formation

Protein then transported to golgi in a vesicle

KDEL receptors recognise lys(K)-asp(D)-glu(E)-leu(L) sequence on proteins that should be in ER = transport back to ER to concentrate here

• Proteins are glycosylated and manose-6 residue phosphorylated (distinct to lysosomal proteins)
• This residue identified by M6P receptor on trans-golgi 
• Receptor changes conformation when binding
• Clathrin binds to cytoplasmic domain - forms mesh-like pit around receptor = vesicle coated in clathrin
• Cathrin subunits dissociate and vesicles form early lysosome w/ low pH
• Low pH = association between manose-6P and receptor lost
• M6P receptor recycled back to golgi
• Transport vesicle formed around protein - becomes lysosome

Golgi packages proteins for apical membrane into seperate vesicles than those to basolateral membrane

Achieved by recpetor surface interactions, similar to lysosomal protein targeting

Vesicles attach to microtubules on cytoskeleton - ATP used to transport to membrane

if proteins attached to wrong membrane - binding pathways aren't present - protein digested by proteasomes/endocytosed 

Both rough ER and cis/medial golgi - ALWAYS BEGINS IN ER

Cardohydrate attachment = wide diversity of proteins

Dolichol lipids deliver oligosaccharide trees (3 glucose, 9 mannose, 2 N-acetylglucosamine) onto asparagine residues as translation occurs

Enzymes in ER/golgi add/remove carbohydrates to specificize

Golgi - protein groups e.g. UDP/CMP carry monosaccharides to tree for addition

Unique glycosylation patterns - critical to protein function

LDL - protein and cholesterol particle

• LDL binds to cluster of LDL receptors
• Forms clathrin occupied pit
• Forms vesicle
• Clatherin dissociated - acidic pH forms in vesicle
• Receptor and LDL particle dissociating
• Receptor recycled
• LDL particle delivered to lysosome
• Degraded using enzymes
• Products released