Low temperature blocks exit of pro-opiomelanocortin from the endoplasmic reticulum but not subsequent delivery to the site of prohormone processing

The effect of reduced temperature on the delivery of the prohormone pro-opiomelanocortin (POMC) to the site of prohormone processing was investigated in the mouse anterior pituitary cell line AtT20. At 20 degrees C processing was substantially inhibited and was almost completely arrested at 18 degrees C. Earlier studies with membrane glycoproteins indicated that at these temperatures protein movement was blocked at the level of exit from the Golgi apparatus. In contrast it was found here that the inhibition of processing at reduced temperature was due to the retention of POMC in the endoplasmic reticulum. When POMC was allowed to progress to the Golgi before temperature was reduced, subsequent processing was only slightly retarded by incubation at 18 degrees C. This indicates either that Golgi exit is not inhibited at this temperature, or that the processing apparatus exists in the Golgi. A surprising incidental result was that when held in the endoplasmic reticulum at low temperature POMC is apparently subject to post-translational N-linked glycosylation.