A HYPOTHESIS ON THE TRAFFIC OF MG160, A MEDIAL GOLGI SIALOGLYCOPROTEIN, FROM THE TRANS-GOLGI NETWORK TO THE GOLGI CISTERNAE

We have reported that MG160, an intrinsic membrane sialoglycoprotein o f the Golgi apparatus (GA), resides in the medial cisternae of the org anelle (Gonatas et al. (1989) J. Biol. Chem. 264, 646-653). In order t o resolve the question whether MG160 acquires sialic acid residues in the trans cisternae or trans-Golgi network (TGN) prior to its retrogra de transport, we have examined the effects of brefeldin A (BFA) on the post-translational processing of MG160, and the distribution of inter nalized wheat germ agglutinin covalently linked with HRP (WGA-HRP), wh ich labels the TGN (Gonatas et al. (1977) J. Cell BioL 73, 1-13). In B FA-treated PC12 cells, MG160 acquires resistance to endo H, but fails to be sialylated. This effect occurs in parallel with the redistributi on of MG160 into an ER compartment dispersed throughout the cytoplasm including the nuclear envelope, and the collapse of the WGA-HRP-labell ed TGN into vesicles and tubules surrounding the centriole. These resu lts suggest that MG160 is not sialylated in BFA-treated cells because it is sequestered from the sialyltransferase enzyme(s), presumably loc ated in the TGN, and provide evidence supporting the hypothesis for a retrograde transport pathway that recycles resident GA proteins, inclu ding MG160, between the Golgi cisternae and the TGN. To examine furthe r the above hypothesis we studied cells treated with BFA and then allo wed to recover from the effect of the drug for various lengths of time . After 15 minutes of recovery, cisternae of the Golgi apparatus, typi cally found in the pericentriolar region, are labeled by both MG160 an d WGA-HRP. Thirty minutes after removal of BFA, the sialylation of MG1 60 has begun and by one hour of recovery the protein has matured to it s final apparent molecular mass. These results are consistent with the hypotheses that under physiologic conditions, either MG160 is sialyla ted in a distal Golgi compartment and then returns to the medial Golgi , or that the sialic acid transferase(s) undergo retrograde transport. Additional morphological evidence of a retrograde pathway is provided by the retrograde flow of internalized WGA-HRP into all of the Golgi cisternae during prolonged exposure to the lectin. Taken together, the se results provide evidence for the existence, under physiologic condi tions, of a retrograde transit pathway active in the distal Golgi appa ratus. This hypothesis may be tested when cloned cDNA for MG160, and a ntibodies specific for rat neural cell sialyltransferases become avail able.