ER membrane proteins Flashcards
type 1
COO- facing cytosol, NH3+ facing lumen
type 1 seq
N term signal seq, stop transfer anchor seq (STA)
type 1 mech
N term signal seq cleaved, elongated (using SRP mech) until STA made, prevents chain from going in farther into lumen, STA moves laterally & anchors in phospholipid bilayer, C term keeps syn into cytosol
type 2/3 seq
signal anchor seq (SA), no cleavable signal seq
type 2
NH3+ facing cytosol, COO- facing lumen
type 3
COO- facing cytosol, NH3+ facing lumen, SA seq near N term so that side is stubby
type 2/3 mech
SA seq syn, bound by SRP, directs to ER mem (SRP mech w/out cleavable signal seq), SA moves laterally & anchors in phospholipid bilayer, C term always syn last, released in the end
how to know which way the protein is oriented?
pos charge always faces cytosol
tail-anchor proteins
NH3+ facing cytosol, COO- facing lumen, HPB seq near C term so that side is stubby
tail-anchor proteins seq
single HPB topogenic seq
tail-anchor proteins mech
C term always syn last, needs a special mech, does not use SRP mech, GET3•ATP + C term tail docks onto GET1/2 (in ER mem), ATP -> ADP, leaves, C term tail released from GET3 & embedded into ER mem, GET3 gets another ATP
type 4/multipass
N term on same side as C term if even # of helices; on opp sides if odd # of helices
type 4/multipass seq
SA and STA
If N term in cytosol, alternates SA/STA
If N term in lumen, starts with SA, then alternates SA/STA
GPI anchor protein structure
HPB portion- fatty acyl chains
HPL portion- carbohydrates and phosphate groups
GPI anchor protein structure mech
protein already syn (type 1 mech), initially inserted into ER mem, lumen part cleaved by GPI transamidase, moved onto preformed GPI anchor
