Oligomerization of a membrane protein correlates with its retention in the Golgi complex

While the specificity of oligomerization was dictated by the sequence of the membrane-spanning region, the cytoplasmic tail of Gml was necessary for SDS-resistance and may play a role in oligomer formation. When the cytoplasmic tails of preexisting oligomers were digested with trypsin, the proteolytic products were soluble in SDS but continued to migrate as oligomers on sucrose gradients. Based on potential trypsin cleavage sites in the cytoplasmic tail of Gml and the migration of the proteolytic fragments on SDS-PAGE, it is possible that removal of the last three carboxyterminal residues of Gml destroys its resistance to solubilization by SDS. Alternatively, trypsin digestion could remove another component of the oligomer that is required for SDS resistance. Interestingly, cell treatment with cytochalasin D blocked the formation of SDS-resistant oligomers, suggesting that the acquisition of SDS resistance may reflect association of the Gml cytoplasmic tail with an actin-based cytoskeleton. We are currently constructing a series of Gml proteins with truncated cytoplasmic tails to determine if these proteins are correctly targeted and form oligomers.