Abstract
COPI coated vesicles carry material between Golgi compartments, but the role of COPI in the secretory pathway has been ambiguous. Previous studies of thermosensitive yeast COPI mutants yielded the surprising conclusion that COPI was dispensable both for the secretion of certain proteins and for Golgi cisternal maturation. To revisit these issues, we optimized the anchor-away method, which allows peripheral membrane proteins such as COPI to be sequestered rapidly by adding rapamycin. Video fluorescence microscopy revealed that COPI inactivation causes an early Golgi protein to remain in place while late Golgi proteins undergo cycles of arrival and departure. These dynamics generate partially functional hybrid Golgi structures that contain both early and late Golgi proteins, explaining how secretion can persist when COPI has been inactivated. Our findings suggest that cisternal maturation involves a COPI-dependent pathway that recycles early Golgi proteins, followed by multiple COPI-independent pathways that recycle late Golgi proteins.
Highlights
The COPI coat was first visualized nearly 30 years ago on vesicles budding from Golgi cisternae (Orci et al, 1986)
The two subcomplexes of yeast COPI localize to the early Golgi
The molecular basis of the thermosensitivity is typically unknown, creating uncertainty about whether the mutant protein has been completely inactivated by the temperature shift
Summary
The COPI coat was first visualized nearly 30 years ago on vesicles budding from Golgi cisternae (Orci et al, 1986). As a Golgi cisterna matures into a trans-Golgi network (TGN) compartment, COPI vesicles bud to remove resident Golgi proteins, thereby helping to drive the Golgi-to-TGN biochemical conversion (Papanikou and Glick, 2014). Both of these predictions about the role of COPI have been tested using the yeast Saccharomyces cerevisiae. This analysis helps to integrate the yeast data into a broader understanding of COPI function
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