ABSTRACT
Mechanisms for exporting variably sized cargo from the endoplasmic reticulum (ER) using the same machinery remain poorly understood. COPII-coated vesicles, which transport secretory proteins from the ER to the Golgi apparatus, are typically 60-90 nm in diameter. However, collagen, which forms a trimeric structure that is too large to be accommodated by conventional transport vesicles, is also known to be secreted via a COPII-dependent process. In this paper, we show that Sec12, a guanine-nucleotide exchange factor for Sar1 guanosine triphosphatase, is concentrated at ER exit sites and that this concentration of Sec12 is specifically required for the secretion of collagen VII but not other proteins. Furthermore, Sec12 recruitment to ER exit sites is organized by its direct interaction with cTAGE5, a previously characterized collagen cargo receptor component, which functions together with TANGO1 at ER exit sites. These findings suggest that the export of large cargo requires high levels of guanosine triphosphate-bound Sar1 generated by Sec12 localized at ER exit sites.
Subject(s)
Antigens, Neoplasm/metabolism , Collagen Type VII/metabolism , DNA-Binding Proteins/metabolism , Endoplasmic Reticulum/metabolism , Guanine Nucleotide Exchange Factors/metabolism , Monomeric GTP-Binding Proteins/metabolism , Neoplasm Proteins/metabolism , Transcription Factors/metabolism , Animals , Antigens, Neoplasm/genetics , Aryl Hydrocarbon Receptor Nuclear Translocator/metabolism , COP-Coated Vesicles/metabolism , Cell Line, Tumor , Collagen Type VII/genetics , DNA-Binding Proteins/genetics , Female , Golgi Apparatus , Guanine Nucleotide Exchange Factors/genetics , Guanosine Triphosphate/metabolism , HEK293 Cells , HeLa Cells , Humans , Jurkat Cells , Neoplasm Proteins/genetics , Protein Transport , RNA Interference , RNA, Small Interfering , Rats , Rats, Wistar , Transcription Factors/genetics , Vesicular Transport Proteins/geneticsABSTRACT
Cutaneous T-cell lymphoma-associated antigen 5 (cTAGE5), an originally identified tumor antigen, is overexpressed in various cancer cell lines. The cDNA encodes an integral membrane protein containing two coiled-coil motifs and a proline-rich domain. We show that cTAGE5 specifically localizes to the endoplasmic reticulum (ER) exit sites. In addition, cTAGE5 forms a complex with TANGO1 (MIA3), a previously characterized cargo receptor for collagen VII, by the interaction of their coiled-coil motifs. Of interest, cTAGE5, as well as TANGO1, is capable of interacting with the inner-layer coatomer of COPII Sec23/24 complex through their C-terminal proline-rich domains and required for collagen VII secretion. We propose that cTAGE5 acts as a coreceptor of TANGO1 for collagen VII export from the ER.
