Fragmentation of a Golgi-localized chimeric protein allows detergent solubilization and reveals an alternate conformation of the cytoplasmic domain.

Golgi resident proteins maintain their localization despite a continual protein and lipid flux through the organelle. To study Golgi retention mechanisms, we have focused upon the chimeric protein Gml. This protein contains the Golgi transmembrane domain targeting signal from the infectious bronchitis virus M protein and the lumenal and cytoplasmic domain of the vesicular stomatitis virus glycoprotein (VSV G). The Gml protein is targeted to the Golgi where it forms an unusually stable detergent-resistant oligomer. The formation of oligomeric structures may aid retention of Golgi resident proteins. Thus, determining the stabilization mechanism may shed light on Golgi protein retention. Previous work determined that the transmembrane domain is required for the targeting and oligomerization of Gml, but it is the cytoplasmic tail that stabilizes the complexes [Weisz, O. A., Swift, A. M., and Machamer, C. E. (1993) J. Cell Biol. 122, 1185-1196]. However, further study of the oligomer has been difficult due to its insolubility. Here we report that fragmenting the Gml protein into several pieces facilitates solubilization by sodium dodecyl sulfate (SDS). By analyzing the fragments produced after cleavage, we determined that the stability of the oligomer is not caused by covalent linkage of Gml to itself or other proteins. The fragment corresponding to the transmembrane domain and tail of Gml had an enhanced mobility in SDS gels relative to the same fragment of the parent VSV G protein. The enhanced migration of the tail fragment does not reflect sequence differences or post-translational modification, but correlates with Golgi localization and oligomerization. We suggest that the enhanced mobility of the Gml tail fragment reflects an altered conformation which serves to stabilize the detergent-resistant oligomers.