Lecture 2 - Vesicles

Types of Coat proteins

COPII, COPI, Clathrin, Retromer

-drive deformation of membranes from flat sheets into vesicles

COPII moves between

ER to golgi

COPI moves between

Golgi to ER + integrity of Golgi complex

Clathrin moves between

Endocytosis from cell surface (to endosome), exit from TGN (trans Golgi network), recycling to TGN **specificity determined by family of ‘adaptor proteins’

Retromer moves between

Endosome to TGN

Sar1

-in COPII

-small GTPase recruited to ER membrane by sec12

-GDP bound and free floating in cytosol

-becomes GTP bound (activated) and embeds in membrane (conformational change allows hydrophobic tail to bind to membrane

-Sar1-GTP recruits Sec23/24

Sec12

-in COPII

Guanine nucleotide exchange factor (GEF) + trans membrane protein

Grabs Sar1, removes GDP, brings down to membrane, allows GTP to bind

Sec23/24

Heterodimer that induces curvature of the ER membrane in COPII

‘Bow tie ‘ shaped

Sec24links to cargo receptor (cargo receptor = transmembrane and bound to cargo)

Helps recruit sec13/31 from cytosol

Sec13/31

Second heterodimer that induced curvature of ER in COPII

-recruited from cytosol by sec23/24

-forms ‘outer coat’

-inturn recruits more sec23/24 —> cumulative addition induces further curvature

-different isoforms exist (in mammals) to select for different cargos

Arf1

-small GTPase in COPI (similar to sar1)

Recruited to Golgi membrane by GEF (GTP bound/activated = inserts myristoyl into Golgi membrane)

-Arf-GEF = periferal membrane protein (unlike sec12 which is transmembrane)

Coatomers

-Art1-GTP recruits 7 coatomers: components of the COPI complex

-ARF GAP proteins may further stimulate the recruitment of the coatomer —> until bud/vesicle formed

CCVs

CCVs = Clathrin coated vesicles

Operate at TGN, endosome and plasma membrane (specificity determined by adaptors)

Each Clathrin subunit composed of 3 light and 3 heavy chains forming a ‘triskelion’ = can spontaneously self assemble

Adaptors in Clathrin

-provide a link between cargo and Clathrin triskelions (Mickey-mouse structure)

Retromer composed of

Sorting nexin proteins (SNX)

Vacuolar protein sorting proteins (VPS)

SNX and VPS recruitment drives membrane curvature

—> produces tubular extensions