AN INVESTIGATION OF THE ROLE OF TRANSMEMBRANE DOMAINS IN GOLGI PROTEIN RETENTION

The single transmembrane domains (TMDs) of the resident glycosylation enzymes of the Golgi apparatus are involved in preventing these protei ns moving beyond the Golgi. It has been proposed that either the TMDs associate, resulting in the formation of large oligomers of Golgi enzy mes, or that they mediate the lateral segregation of the enzymes betwe en lipid microdomains. Evidence for either type of interaction has bee n sought by examining the retention of sialyltransferase (ST), an enzy me of the mammalian trans Golgi, No evidence could be obtained for spe cific interactions or 'kin recognition' between ST and other proteins of the trans Golgi, Moreover, it is shown that the previously describe d kin recognition between enzymes of the medial Golgi involves the lum enal portions of these proteins rather than their TMDs, To investigate further the role of the ST TMD, the effects on Golgi retention of var ious alterations in the TMD were examined. The addition or removal of residues showed that the efficiency of retention of ST is related to T MD length. Moreover, when a type I plasma membrane protein was express ed with a synthetic TMD of 23 leucines it appeared on the cell surface , but when the TMD was shortened to 17 leucines accumulation in the Go lgi was observed. These observations are more consistent with lipid-ba sed sorting of ST TMD, but they also allow for reconciliation with the kin recognition model which appears to act on sequences outside of th e TMD.