A microtubule-independent role of p150glued in secretory cargo concentration at endoplasmic reticulum exit sites

Abstract

Newly synthesized proteins are sorted into COPII-coated transport carriers at the endoplasmic reticulum (ER). Assembly of the COPII coat complex, which occurs at ER exit sites (ERES), is initiated by membrane association and GTP loading of SAR1, followed by the recruitment of the SEC23/24 and SEC13/31 sub-complexes. Both of these two sub-complexes stimulate GTP hydrolysis and coat disassembly. This inherent disassembly capacity of COPII complexes needs to be regulated to allow sufficient time for cargo sorting and transport carrier formation. Using fluorescence recovery after photobleaching (FRAP) and mathematical modelling we show that p150glued, a component of the dynactin complex, stabilizes the COPII pre-budding complex on ER membranes in a microtubule-independent manner. Concentration of the secretory marker ts-O45-G at ERES is reduced in the presence of a C-terminal p150glued fragment that prevents binding of endogenous p150glued to SEC23. A similar cargo reduction is observed upon p150glued knockdown. Altogether, our data suggest that cargo concentration at ERES is regulated by p150glued to coordinate protein sorting and transport carrier formation with the subsequent long-range transport towards the Golgi complex along microtubules.