ERES (ER exit sites) and the "secretory unit concept"

Area of Science:

• Plant cell biology
• Molecular and Cellular Biology
• Biochemistry

Background:

• The plant Golgi apparatus comprises numerous mobile stacks interacting with the cortical ER.
• Transport between the ER and Golgi is mediated by COP-coated vesicles.
• Previously, ER exit sites (ERES) were identified by COPII protein localization, suggesting association with Golgi stacks.

Purpose of the Study:

• To investigate the precise localization of COPII proteins in higher plants.
• To re-evaluate the existing model of ER-Golgi transport and the concept of the "secretory unit".

Main Methods:

• High-resolution confocal laser scanning microscopy (CLSM) of tobacco leaf epidermis.
• Utilized a novel COPII marker and YFP-SEC24, a known COPII coat protein.

Main Results:

• COPII fluorescence was observed associated with Golgi stacks, not the ER surface.
• This fluorescence likely represents transient accumulation of COPII vesicles within the Golgi matrix before cis-Golgi fusion.
• Calculations indicate a low number of COPII vesicles (<20) are sufficient for the observed signal.

Conclusions:

• The findings challenge the established view of ER exit sites being intimately associated with Golgi stacks.
• The data suggests the