LDLC ENCODES A BREFELDIN-A SENSITIVE, PERIPHERAL GOLGI PROTEIN REQUIRED FOR NORMAL GOLGI FUNCTION

Two genetically distinct classes of low density lipoprotein (LDL) rece ptor-deficient Chinese hamster ovary cell mutants, ldlB and ldlC, exhi bit nearly identical pleiotropic defects in multiple medial and trans Golgi-associated processes (Kingsley, D., K. F. Kozarsky, M. Segal, an d M. Krieger. 1986. J. Cell Biol. 102:1576-1585.). In these mutants, t he synthesis of virtually all N- and O-linked glycoproteins and of the major lipid-linked oligosaccharides is abnormal. The abnormal glycosy lation of LDL receptors in ldlB and ldlC cells results in their dramat ically reduced stability and thus very low LDL receptor activity. We h ave cloned and sequenced a human cDNA (LDLC) which corrects the mutant phenotypes of ldlC, but not ldlB, cells. Unlike wild-type CHO or ldlB cells, ldlC cells had virtually no detectable endogenous LDLC mRNA, i ndicating that LDLC is likely to be the normal human homologue of the defective gene in ldlC cells. The predicted sequence of the human LDLC protein (ldlCp, similar to 83 kD) is not similar to that of any known proteins, and contains no major common structural motifs such as tran smembrane domains or an ER translocation signal sequence. We have also determined the sequence of the Caenorhabditis elegans ldlCp by cDNA c loning and sequencing. Its similarity to that of human ldlCp suggests that ldlCp mediates a well-conserved cellular function. Immunofluoresc ence studies with anti-ldlCp antibodies in mammalian cells established that ldlCp is a peripheral Golgi protein whose association with the G olgi is brefeldin A sensitive. In ldlB cells, ldlCp was expressed at n ormal levels; however, it was not associated with the Golgi. Thus, a c ombination of somatic cell and molecular genetics has identified a pre viously unrecognized protein, ldlCp, which is required for multiple Go lgi functions and whose peripheral association with the Golgi is both LDLB dependent and brefeldin A sensitive.