Cells that secrete foreign proteins

Regulation of alpha 1-acid glycoprotein externalization and intracellular accumulation in glucocorticoid-induced rat hepatoma cells

alpha 1-Acid glycoprotein (alpha 1-AGP) is a glucocorticoid inducible gene product that is synthesized and secreted by certain rat hepatoma tissue culture (HTC) cell lines such as M1.54. Exposure to monensin, a Na+-K+ ionophore, causes a significant redistribution of alpha 1-AGP into two distinct fractions; immunoprecipitation of [35S]methionine-labeled proteins revealed that a 27% decrease in secretion accounts for a sixfold increase in accumulation of a stable intracellular species. The new intracellular alpha 1-AGP is more heterogeneous than normal while the extra-cellular form is 6000 Da smaller than normal. These effects are due to selective alterations in carbohydrate maturation; endo-beta-N-acetylglucosaminidase H (endo H) digestion demonstrated that both alpha 1-AGP species contain variable numbers of endo H-resistant oligosaccharide side chains ranging between zero and five. Ricin affinity chromatography revealed that the attachment of galactose residues is strikingly correlated with alpha 1-AGP externalization while neuraminidase digestions demonstrated that sialic acid attachment appears unessential for its secretion. Taken together, our results suggest that in the presence of monensin the cellular transport of intracellular destined and externalized alpha 1-AGP proceeds in common through the early segments of the Golgi and at a point prior to or at the compartment containing galactosyl transferase, alpha 1-AGP becomes committed for secretion.

Efficient expression of cloned complementary DNAs for secretory proteins after injection into Xenopus oocytes

Cloned complementary DNAs encoding chicken ovalbumin, chicken prelysozyme and calf preprochymosin, prochymosin and chymosin were inserted downstream from various viral promoters in modified recombinant "shuttle" vectors. Microinjection of the ovalbumin, prelysozyme and preprochymosin constructs into the nuclei of Xenopus laevis oocytes resulted in the synthesis, segregation in membranes and secretion into the extracellular medium of ovalbumin, lysozyme and prochymosin, respectively. Judging from molecular weight estimations, lysozyme and prochymosin were correctly proteolytically processed while ovalbumin, which lacks a cleavable signal sequence, was glycosylated. Injection of the DNA construct encoding prochymosin without its signal sequence resulted in synthesis of prochymosin protein that was localized exclusively in the oocyte cytoplasm. No immunospecific protein was detected after injection of the DNA encoding mature chymosin. In terms of protein expression in oocytes, the Herpes simplex thymidine kinase (TK) promoter was up to sevenfold more effective than the simian virus 40 (SV40) early promoter, and equally as effective as the Moloney murine sarcoma virus long terminal repeat element. Where tested, protein expression in oocytes was much reduced if DNA sequences encoding the SV40 small t intron and its flanking sequences were present in the constructs. S1 nuclease mapping of transcripts produced after injection of DNAs containing the TK promoter indicated that the majority of transcripts initiated at, or within, two bases of the known "cap" site. However, minor transcripts initiating upstream from this site were observed and one (or more) of these transcripts was responsible for the synthesis of an ovalbumin polypeptide containing a 51 amino acid N-terminal extension. This extended protein remained in the oocyte cytosol. When ovalbumin cDNA was inserted into the vectors with opposite polarity to the viral promoter, expression in oocytes resulted in the predominant synthesis and secretion of a variant ovalbumin with a 21 amino acid N-terminal extension, although some full-length ovalbumin was also synthesized and secreted. S1 mapping revealed the presence, in these oocytes, of transcripts of predicted polarity initiating 118 bases upstream from the wild type ovalbumin initiator ATG, at a previously unreported SV40 "promoter". No protein synthesis was detected after the injection of these reverse-orientation constructs into baby hamster kidney (BHK-21) cells.

The oocyte as a secretory cell

Xenopus laevis oocytes secrete a large variety of foreign secretory proteins after the microinjection of mRNA or DNA. Two classes of such proteins are discussed in detail. These are the chick oviduct proteins ovalbumin and lysozyme, and the mouse MOPC 21 immunoglobulin. The injection of mRNAs for mouse immunoglobulin heavy or light chain leads to the synthesis, segregation, but not secretion of the encoded proteins unless the two mRNAs are simultaneously or sequentially injected into the same oocytes. Chicken ovalbumin and lysozyme are synthesized and secreted from oocyte after the injection of either oviduct mRNA or cloned DNA (ovalbumin). The secreted lysozyme is exported considerably faster than ovalbumin; however, 40% of the lysozyme synthesized cannot be secreted and, after fractionation of oocytes on sucrose gradients, is found in a higher density position than ovalbumin. No competition at the level of secretion or translation was noted when different amounts of immunoglobulin and ovalbumin mRNAs were injected into oocytes. However, the co-injection of ovalbumin mRNA and mRNAs encoding anti-ovalbumin immunoglobins resulted in the formation of a complex of the two types of protein within the oocyte. In these circumstances, secretion of the immunoglobulin was severely reduced.