Protein segregation in peripheral 15ºC intermediates in response to caffeine treatment

Previous studies have shown that caffeine treatment at 20 degrees C causes the intermediate compartment protein p58 to redistribute from the Golgi region without affecting the localization of the Golgi stack protein mannosidase II (J. Jäntti, E. Kuismanen, J. Cell Biol. 120, 1321-1335 (1993). Here we have dissected further the effect of caffeine on transport of Golgi and intermediate compartment proteins from the cell periphery to the perinuclear Golgi region. To accumulate proteins in the peripheral membranes, BHK-21 cells were treated with brefeldin A to redistribute marker proteins towards the ER. Following BFA wash-out and subsequent incubation at 15 degrees C, p58, the coat protein beta-COP, and Man II were all localized in the peripheral 15 degrees C-intermediates. When the cells were shifted from 15 degrees C to 20 degrees C all the proteins were recentralized to the Golgi region. However, if the temperature shift was carried out in the presence of 10 mM caffeine, p58 and beta-COP maintained their peripheral localization, whereas Man II was transported to the Golgi region. The results indicate that caffeine at 20 degrees C does not block the centralization of Man II from peripheral sites to the central Golgi region. Therefore, its effect on ER to Golgi transport appears to be manifested specifically at ER exit. Furthermore, our results indicate that segregation of intermediate compartment and Golgi stack proteins can occur at the level of the peripheral 15 degrees C-intermediates. Immunoelectron microscopic localization of p58 and Man II showed that these peripheral intermediates consisted of tubules and small stacks of cisternae. Within the tubular intermediates both p58 and Man II appeared to segregate to membrane subdomains. Finally, examination of serial and thick sections support the idea that the stacked structures can be generated from tubular intermediates.