ne102 4/27 lecture: membrane transport + fusion; microtubules

golgi apparatus

stack of flattened membrane-enclosed sacs

how to carry proteins to intracellular compartments?

vesicles that form from one membrane can fuse w/ another; vesicle from ER → golgi → endocytosis or exocytosis (transport vesicles containing soluble proteins)

glycosylation

occurs in ER + golgi; in golgi vesicle: growing polypeptide chain in cytosol, PTM where oligosaccharide binds to Asn in ER lumen → moves to extracellular space while cytosolic stays

exocytosis

soluble proteins can be secreted outside the cell; transmembrane proteins are not exocytosed by remain inserted in plasma membrane

constitutive/unregulated pathway exocytosis

trans golgi network → newly synthesized soluble proteins for constitutive secretion + plasma membrane lipids + protein → released (ex: APP)

regulated pathway exocytosis

trans golgi network → secretory vesicle storing secretory proteins → extracellular signal (hormone, ntm) drives signal transduction → released (ex: BDNF)

endocytosis

vesicle buds from plasma membrane → carries vesicles’ content to intracellular organelles

how do vesicles form? is ts the same for exo and endo?

regions of membrane are pinched off from larger membranes forming rounded vesicles; yes

2 types of vesicles

1. COP-coated vesicles: ER to golgi + back

2. clathrin-coated vesicles: golgi to PM + back to endosomes

clathrin coated vesicle process

cargo receptor + molecule → adaptin binds → clathrin forms basket like molecule to carry macromolecules, like receptors → dynamin pinches + forms clathrin coated vesicle → clathrin/adaptin uncoat → naked transport vesicle

what helps direct transport vesicles to target membranes?

Rab + SNARE proteins; specific Rab proteins will make interactions only with specific tethering proteins at target membranes (Rab27 in neurons)

snare proteins

regulate fusion of vesicles with PMs

1. docking: v snares (VAMP2) on transport vesicle tangle with t snares (SNAP25, syntaxin 1) on target membrane

2. membrane fusion: lipid bilayers fuse

microtubules

hollow tubes of tubulin: alpha-beta tubulins form dimer → fuse to form protofilament → 13 parallel protofilaments form microtubule w/ lumen middle

how is growth of microtubules organized?

centrosomes made of gamma tubulin + pair of centrioles → alpha tubulin end (-) binds to gamma tubulin → beta tubulin (+) grows out (polarity is KEY for protein transport)

dynamic instability of microtubules

controlled by GTP hydrolysis:

growth: GTP bound to beta subunit added to growing end of microtubule → addition is faster than GTP hydrolysis, or no GTP hydro = growth towards + end

shrink: protofilaments containing GDP tubulin peel away from wall → GDP tubulin released into cytosol → shrink (GTP hydrolysis to GDP = shrink)

kinesins

made of cargo + tail + globular head, moves to synaptic terminal to plus end on microtubules = anterograde

dyneins

made of cargo + tail + globular head, moves to cell body to neg end on microtubules = retrograde