A number of Golgi glycosyltransferases has been cloned to date. They all are membrane proteins and share the same type II topology, but they do not possess an obvious sequence homology which would suggest a common Golgi retention signal. However, it was shown that the membrane-spanning domain and its flanking regions contain necessary and sufficient information for Golgi retention of these enzymes. Currently, two mutually complementary models have been proposed to explain the mechanism of Golgi retention of glycosyltransferases mediated by their transmembrane domain. The first model postulates the retention through oligomerization, which prevents enzymes from entering the transport vesicles. The second suggests that retention depends on the length of a membrane-spanning domain and thickness of the membrane along the Golgi complex. It has to be pointed out that neither the oligomerization nor the membrane thickness model alone can answer all questions and further work is still needed to elucidate the retention process of Golgi proteins.
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