A GTP Hydrolase Activity Purified from Transitional Endoplasmic Reticulum of Rat Liver Binds Retinol

Abstract

GTP hydrolysis by an endoplasmic reticulum fraction from rat liver enriched in part-rough, part-smooth transition elements (transitional endoplasmic reticulum is inhibited by all-trans retinol (Zhao et al. 1990). The inhibition is non-competitive and the Km for GTP is 0.3 mM. The inhibitory effect is most apparent with GTP as substrate. Retinol did not significantly inhibit hydrolysis of the nucleoside diphosphates or of ATP or UTP. A GTP hydrolase enriched fraction from transitional endoplasmic reticulum of rat liver binds retinol. This protein may fulfill some role in mediating intracellular interactions, such as regulation of membrane traffic. GTP-binding proteins appear to be required for most vesicle formation-fusion events in eukaryotic secretory pathways (Bourne 1988). Within the cell, the transfer of materials from the endoplasmic reticulum to the Golgi apparatus occurs via transition vesicles. Retinol stimulates formation of transition vesicles, but not their fusion with the cis Golgi apparatus in a cell-free system from rat liver (Nowack et al. 1990). This study further characterizes the GTP hydrolase that is inhibited by retinol.