(Abstract of my PhD thesis)

Cauliflower mosaic virus is a pararetrovirus that forms icosahedral particles. Three different forms of the capsid protein, all processed from the 57 kDa capsid protein precursor, the product of ORF IV, are found in purified virions.


Schematic drawing of ORF IV and one of its derivatives, p44

Both termini of the capsid protein precursor and the N-terminus of the largest processed form (p44) contain signals leading to protein degradation. These were mapped by fusion to chloramphenicol acetyl transferase using a modified ubiquitin protein reference technique. The N-terminus of p44 contains a degradation motif characterised by proline-, glutamate-, aspartate-, serine-, and threonine-residues (PEST), which could be inactivated by mutation of three glutamic acid residues to alanines. When introduced into the CaMV genome, these mutations caused a delay in virus spread. A partial revertant was detected after three passages, suggesting that those Glu residues play an important role in the virus life cycle. The signals from the precursor do not correspond to known degradation motifs, although they have a high degradation activity in plant protoplasts. All three instability determinants were active in HeLa cells, but the PEST signal had a significantly higher degradation activity there, whereas the precursor signals were less active. Inhibition studies suggest that the signal within the N-terminus of the precursor is targeting to the proteasome.