The oocyte of Xenopus laevis has been established as a system with which to study the export of many secretory proteins. When oocytes are micro-injected with hen oviduct mRNA, ovalbumin and lysozyme accumulate at different rates in the surrounding incubation medium. This thesis concerns an investigation of the molecular and cellular basis for this non-parallel secretion. Kinetic studies confirm that the intrinsic rate of lysozyme secretion from oocytes is approximately twelve times that of ovalbumin and show that not all of the lysozyme is available for export. This slower rate of ovalbumin export is maintained following injection of a range of concentrations of oviduct mRNA or of purified ovalbumin messenger, the latter having been obtained by hybridisation to cloned ovalbumin complementary DNA. These results suggest that the differential rates of secretion of these two proteins observed in oocytes are not the consequence of competition for amphibian or avian factors and show that oviduct-specific proteins are not required for ovalbumin secretion. To analyse the cellular basis for this non-parallel secretion, oocyte-fractionation protocols have been employed: if organelles of the secretory pathway could be separated, then it might be possible to distinguish between two possibilities. Firstly, that this non-parallel secretion reflects the relative retardation of ovalbumin during stages preceding the formation of the final exocytotic vesicles (containing both ovalbumin and lysozyme). Secondly, the two proteins might be segregated from one another and secreted by different tracks along the same pathway. Microsomal fractions containing oviduct proteins have been isolated from oocytes using sucrose gradients, but a demonstrable separation of Golgi- and ER-derived microsomal subfractions has not been obtained. Analysis of pulse-labelIed and pulse-chased oocytes on sucrose gradients show that non-secretory lysozyme slowly enters a novel high density oocyte compartment. Movement of lysozyme into this compartment may be prevented by the use of monensin.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:331700 |
| Date | January 1982 |
| Creators | Cutler, Daniel Franklin |
| Publisher | University of Warwick |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://wrap.warwick.ac.uk/110299/ |
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