Intracellular movement of two mannose 6-phosphate receptors: return to the Golgi apparatus.

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Abstract
We have used Chinese hamster ovary (CHO) cells and a murine lymphoma cell line to study the recycling of the 215-kD and the 46-kD mannose 6-phosphate receptors to various regions of the Golgi to determine the site where the receptors first encountered newly synthesized lysosomal enzymes. For assessing return to the trans-most Golgi compartments containing sialyltransferase (trans-cisternae and trans-Golgi network), the oligosaccharides of receptor molecules on the cell surface were labeled with [3H]galactose at 4.degree. C. Upon warming to 37.degree. C, the [3H]galactose residues on both receptors were substituted with sialic acid with a 51/2 .apprx. 3 hrs. Other glycoproteins acquired sialic acid at least 8-10 times slower. Return of the receptors of the trans-Golgi cisternae containing galactosyltransferase could not be detected. Return to the cis/middle Golgi cisternae containing .alpha.-mannosidase I was measured by adding deoxymannojirimycin, a mannosidase I inhibitor, during the initial posttranslational passage of [3H]mannose-labeled glycoproteins through the Golgi, thereby preserving oligosaccharides which would be substrates for .alpha.-mannosidase I. After removal of the inhibitor, return to the early Golgi with subsequent passage through the Golgi complex was measured by determining the conversion of the oligosaccharides from high mannose to complex-type units. This conversion was very slow for the receptors and other glycoproteins (t1/2 .apprx.20 h). Exposure of the receptors and other glycoproteins to the dMM-sensitive .alpha.-mannosidase without movement through the Golgi apparatus was determined by measuring the loss of mannose residues from these proteins. This loss was also slow. These results indicate that both Man-6-P receptors routinely return to the Golgi compartment which contains sialyltransferase and recycle through other regions of the Golgi region less frequently. We infer that the trans-Golgi network is the major site for lysosomal enzyme sorting in CHO and murine lymphoma cells.