RETARDATION OF A SURFACE PROTEIN CHIMERA AT THE CIS GOLGI

Dipeptidyl peptidase IV (D4) and the alpha subunit of human chorionic gonadotrophin (alpha hcg) are plasma membrane and secretory proteins, respectively. In the course of studies to understand mechanisms involv ed in transport along the exocytotic pathway, the ectoplasmic domain o f D4 was replaced by the mature polypeptide of alpha hcg, resulting in the membrane anchored chimera, D4 alpha hcg. Surprisingly, when trans fected into Chinese hamster ovary (CHO) and Madin-Darby canine kidney (MDCK) cells, strong perinuclear Golgi staining was predominant, in ad dition to the expected surface staining. By following the biogenesis a nd transport of the molecule, it was established that newly synthesize d D4 alpha hdg is eventually transported to the cell surface but only after a significant retardation in the Golgi apparatus. The compartmen t of retardation was identified as the early or cis Golgi, before the medial Golgi, where resistance to endoglycosidase (endo) H is conferre d. As a result of the transport retardation of the chimera, we were ab le to document the appearance of an endo D sensitive intermediate, whi ch is usually too transient to be apparent in normal cells. The retard ation of this chimera in the cis Golgi complements our previous report in which the D4 molecule with its transmembrane domain replaced by th at of aminopeptidase N resulted in retardation in the trans Golgi/tran s Golgi network in MDCK and CHO cells [Low, S. H., Tang, B. L., Wong, S. H., and Hong, W. (1994) J. Biol. Chem. 269, 1985-1994). Together, t hese reports indicate that transport along the exocytic pathway may no t be simply by default but requires some sort of signal, the disruptio n of which results in inefficient intra-Golgi and/or Golgi to surface transport.