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
advantage of GPI anchor
able to move laterally in membrane, can diffuse relatively rapidly in the phospholipid bilayer membrane
