Bowers, Katherine Elisabeth; (1998) Sorting of CD4 and the SIV envelope glycoprotein in the endocytic pathway. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Two main classes of molecular signals for clathrin-mediated endocytosis have been identified: tyrosine-based and dileucine-based. To understand the modes of action of these signals in more detail, I have analysed the envelope protein of SIV, which contains a tyrosine-based signal, and CD4, which contains a dileucine-based signal. I have determined that the membrane-proximal tyrosine-based internalisation signal in the cytoplasmic domain of the SIV envelope transmembrane protein (SIV-TM) is part of a YXX0 motif. I have also identified and mapped the approximate location of an additional endocytosis signal or signals in SIV-TM. The efficient endocytosis of SIV-TM, mediated by more than one endocytosis signal, has implications for viral assembly and pathogenesis. Tyrosine- and dileucine-based endocytosis signals are thought to function by interaction with clathrin-associated adaptor complexes. I was unable to demonstrate the binding of CD4 or SIV-TM to the AP2 adaptor using co-immunoprecipitation techniques or the yeast 2 hybrid system. However, preliminary experiments using surface plasmon resonance suggest that both CD4 and SIV-TM do bind AP2. In addition to sorting at the plasma membrane, molecules such as CD4 are sorted within endosomes. COPI coat proteins have recently been implicated in protein transport from early to late endosomes. I have used a cell line with a temperature sensitive mutation in the COPI protein eCOP, to investigate the role of eCOP in CD4 sorting. In the eCOP mutant cells, protein transport from early to late endosomes is inhibited. Exposure of eCOP mutant cells to stress also causes an interesting phenotype. The mutant cells appear able to accumulate a fluid phase marker in an early endosome compartment, and show an apparent inability to recycle a bulk membrane marker.

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