Isolation of Trypanosoma brucei variant specific antigen mRNA by immunoprecipitation of polysomes

Purification of Messenger Ribonucleoprotein Particles via a Tagged Nascent Polypeptide

The cytoplasmic fates of mRNAs are influenced by interactions between RNA-binding proteins and cis regulatory motifs. In the cytoplasm, mRNAs are present as messenger ribonucleoprotein particles, which include not only proteins that bind directly to the mRNA, but also additional proteins that are recruited via protein-protein interactions. Many labs have sought to purify such particles from cells, with limited success. We here describe a simple two-step procedure to purify actively translated mRNAs, with their associated proteins, from polysomes. We use a reporter mRNA that encodes a protein with three streptavidin binding peptides at the N-terminus. The polysomal reporter mRNA, with associated proteins, is purified via binding to a streptavidin matrix. The method takes four days, and can be applied in any cell that can be genetically manipulated. Using Trypanosoma brucei as a model system, we routinely purified 8% of the input reporter mRNA, with roughly 22-fold enrichment relative to un-tagged mRNAs, a final reporter-mRNA:total-mRNA ratio of about 1:10, and a protein purification factor of slightly over 1000-fold. Although the overall reporter mRNP composition is masked by the presence of proteins that are associated with many polysomal mRNAs, our method can be used to detect association of an RNA-binding protein that binds to specifically to a reporter mRNA.

The biosynthesis of Trypanosoma brucei variant surface glycoproteins--in vitro processing of signal peptide and glycosylation using heterologous rough endoplasmic reticulum vesicles

The ability of stripped microsomal dog pancreas vesicles to process precursors of the variant glycoprotein of Trypanosoma brucei has been investigated. The vesicles were shown to be capable of cleaving a peptide precursor variant surface glycoprotein, identified as the N-terminal signal peptide in the case of one variant. The vesicles were also shown to add carbohydrate to precursor variant surface glycoprotein. This carbohydrate, which was sensitive to the action of endo-H was located within a C-terminal CNBr fragment in one variant. However, the vesicles were not capable of adding the oligosaccharide carrying the variant surface glycoprotein cross-reacting determinant.

Characterization of the DNA duplication-transposition that controls the expression of two genes for variant surface glycoproteins in Trypanosoma brucei

The genome of Trypanosoma brucei carries over a hundred genes coding for different variants of the major surface glycoprotein. Activation of some of these genes is accompanied by a duplication and transposition of the gene (the basic copy) to another region in the genome where it is transcribed. We present here physical maps of the basic and transposition-activated genes for two surface glycoproteins of Trypanosoma brucei, stock 427. In both cases the transposed segment starts 1-2 kb in front of the coding region and ends within the 3'-terminal region of the gene. The DNA segments flanking both transposed genes are indistinguishable and share a 6-kb stretch upstream and a 8-kb stretch downstream of the transposed segment not cut by several restriction endonucleases. The 5' borders of the two transposed segments are homologous and contain sequences present in many copies in the genome. A different repeated sequence has previously been found at the 3' edge of the transposed segment. The replicative transposition may, therefore, involve a unidirectional gene conversion initiated by base pairing between the edges of the transposed sequence and a single expression site elsewhere in the genome.

Biosynthesis of Trypanosoma brucei variant surface glycoprotein. I. Synthesis, size, and processing of an N-terminal signal peptide

An N-terminal signal peptide has been identified on a precursor of the variant surface glycoprotein (VSG) of Trypanosoma brucei, synthesised both in vitro and in vivo. An ordered cyanogen bromide (CNBr) cleavage map was constructed for both mature VSG and for precursor VSG, and this allowed a size estimate of the signal peptide to be made by comparison of the N-terminal CNBr peptides produced from each. The signal peptide contains 30-40 amino acids. Analysis of the precursor VSGs produced when nascent polypeptide chains on T. brucei polysomes were completed in the absence of reinitiation showed that the signal peptide is removed before translation is complete and that the same translational start signal is utilised in the trypanosome and in the rabbit reticulocyte protein synthesis system.