The protein bodies of the endosperm of the seeds of the castor oil plant Ricinus cannon is contain a variety of storage proteins, along with two lectins, ricin and Ricinus communis agglutinin. Ricin is a heterodimer consisting of an A chain which is toxic to cell-free translation systems, and a B chain which has galactose-binding activity; the whole molecule is toxic to cells and animals by virtue of the ability of the A chain to enzymatically inactivate ribosomes after it crosses the cell membrane, this latter being achieved after binding of the molecule to cell surfaces by the B chain. The agglutinin consists of two ricin-like species linked non-covalently, is divalent, and is not significantly toxic to cells. Previous work on the synthesis of the lectins indicated diet each subunit had its own precursor, each being co-translationally segregated and glycosylated, and assembly of the lectins was thought to occur after transport to the protein bodies. However, the putative B chain precursor was far larger than the mature B chains, and further protein-based evidence Indicated that It contains both A and B chain sequences. The former A chain precursor was Identified as an albumin which contaminated lectin preparations used for raising antisera. The work reported here confirms that the putative B chain precursor contains A and B chain sequences, by means of the cloning of cDNA complementary to lectin-specific mRNA. Clones of nearly full length have been obtained and sequenced, and the precursor is shown to have an N-terminal signal sequence, which is followed by the A chain sequence, and then by the B chain sequence. A linker of 12 amino acids is shown to be present between the two chains. Sequences corresponding to both lectins are reported, and the similarities and differences between them are discussed. The sequences are placed into context by comparison with other plant nucleotide and protein sequences. Literature on the castor bean lectins is reviewed, and the uses of the clones are discussed, with special reference to their possible use in immunotoxins for cancer therapy.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:353066 |
| Date | January 1984 |
| Creators | Lamb, Francis Ian |
| Publisher | University of Warwick |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://wrap.warwick.ac.uk/112025/ |
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