Modulation of transition vesicle formation and function by vitamin A

Alterations in the glycosylation patterns of glycoproteins can be correlated with vitamin A excess or deficiency. The locus for the action of the vitamin A effects on glycosylation appears to be at the endoplasmic reticulum/Golgi apparatus region of the cell and, more specifically, to target transition vesicles. A cell-free system has been developed that reconstitutes the transition vesicle formation and function that occurs in that region in vitro. A retinol response was sought in the cell-free system that might duplicate the in vivo response to vitamin A. Part-rough, part-smooth transitional endoplasmic reticulum elements, when incubated with ATP and a cytosolic fraction at 37$\sp\circ$C, formed membrane blebs resembling transition vesicles seen in vivo. These membranes, metabolically labeled, transferred radioactive membrane proteins to unlabeled Golgi apparatus (acceptor) immobilized on nitrocellulose strips. Retinol at an optimum concentration of 1 $\mu$g/ml stimulated transfer by more than two-fold. Transition vesicles were isolated by preparative free-flow electrophoresis to separate the transfer process into two steps: ATP-dependent vesicle formation and ATP-independent vesicle fusion. The primary effect of retinol was on the ATP-dependent formation of transition vesicles from endoplasmic reticulum. Three possible mechanisms for the action of retinol on the cell-free transfer of radiolabeled proteins include (1) increase in membrane fluidity of the transitional elements due to the influence of vitamin A, (2) direct stimulation of the ATP-requiring enzyme and (3) prevention of oxidation of the transition vesicles by vitamin A. Modification by vitamin A administration, both in vivo and in vitro, provides one mechanism whereby transition vesicle numbers may be regulated to alter transport through the Golgi apparatus and subsequent glycosylation reactions in the Golgi apparatus