The trans-Golgi network GRIP-domain proteins form alpha-helical homodimers

A recently described family of TGN (trans-Golgi network) proteins, all of which contain a GRIP domain targeting sequence, has been proposed to play a role in membrane transport. On the basis of the high content of heptad repeats, GRIP domain proteins are predicted to contain extensive coiled-coil regions that have the potential to mediate protein-protein interactions. Four mammalian GRIP domain proteins have been identified which are targeted to the TGN through their GRIP domains, namely p230, golgin-97, GCC88 and GCC185. In the present study, we have investigated the ability of the four mammalian GRIP domain proteins to interact. Using a combination of immunoprecipitation experiments of epitope-tagged GRIP domain proteins, cross-linking experiments and yeast two-hybrid interactions, we have established that the GRIP proteins can self-associate to form homodimers exclusively. Two-hybrid analysis indicated that the N- and C-terminal fragments of GCC88 can interact with themselves but not with each other, suggesting that the GRIP domain proteins form parallel coiled-coil dimers. Analysis of purified recombinant golgin-97 by CD spectroscopy indicated a 67% alpha-helical structure, consistent with a high content of coiled-coil sequences. These results support a model for GRIP domain proteins as extended rod-like homodimeric molecules. The formation of homodimers, but not heterodimers, indicates that each of the four mammalian TGN golgins has the potential to function independently.