RETENTION OF GLYCOSYLTRANSFERASES IN THE GOLGI-APPARATUS

A number of Golgi glycosyltransferases has been cloned to date. They a ll are membrane proteins and share the same type II topology, but they do not possess an obvious sequence homology which would suggest a com mon Golgi retention signal, However, it was shown that the membrane-sp anning domain and its flanking regions contain necessary and sufficien t information for Golgi retention of these enzymes. Currently, two mut ually complementary models have been proposed to explain the mechanism of Golgi retention of glycosyltransferases mediated by their transmem brane domain The first model postulates the retention through oligomer ization, 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 me mbrane thickness model alone can answer all questions and further work is still needed to elucidate the retention process of Golgi proteins.