A major tyrosine-phosphorylated protein of Trypanosoma brucei is a nucleolar RNA-binding protein.
We have previously identified a set of tyrosine-phosphorylated proteins with apparent molecular masses of 44-46 kDa as some of the major tyrosine phosphorylated species in the protozoan parasite Trypanosoma brucei. We now show that these molecules, herein named Nopp44/46, are localized in the nucleolus. Using monoclonal antibodies, we have isolated Nopp44/46 cDNA clones from expression libraries. Sequence analysis reveals that the predicted amino acid sequence of the molecule is composed of an N-terminal unique region, an internal acidic region, and C-terminal repeat region. Analysis of the cDNA clones and genomic Southern analysis indicated that Nopp44/46 belongs to a multigene family in which different gene copies are very similar but vary in the number of repeats. Interestingly, the repetitive amino acid sequence motif contains multiple RGG (Arg-Gly-Gly) boxes characteristic of RNA-binding proteins. In vitro binding experiments demonstrated that Nopp44/46 is indeed capable of binding nucleic acids. Competition experiments with different RNA homopolymers demonstrated that Nopp44/46 preferentially binds to poly(U). These studies suggest that Nopp44/46 may play a role in RNA metabolism in trypanosomes and raise the possibility that tyrosine phosphorylation may regulate the process.
Das
A
A
Seattle Biomedical Research Institute, Seattle, Washington 98109, USA.
Peterson
G C
GC
Kanner
S B
SB
Frevert
U
U
Parsons
M
M
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D002466:Cell Nucleolus / Q000737:chemistry*; D003001:Cloning, Molecular; D004277:DNA, Single-Stranded / Q000378:metabolism; D017125:Genes, Protozoan; D007150:Immunohistochemistry; D008969:Molecular Sequence Data; D009687:Nuclear Proteins; D010750:Phosphoproteins / Q000737:chemistry* / Q000378:metabolism; D019000:Phosphotyrosine / Q000737:chemistry; D015800:Protozoan Proteins / Q000737:chemistry* / Q000378:metabolism; D016601:RNA-Binding Proteins / Q000737:chemistry* / Q000378:metabolism; D015183:Restriction Mapping; D014346:Trypanosoma brucei brucei / Q000737:chemistry*
D004277:DNA, Single-Stranded; C100391:NOPP44-46 protein, Trypanosoma brucei; D009687:Nuclear Proteins; D010750:Phosphoproteins; D015800:Protozoan Proteins; D016601:RNA-Binding Proteins; D019000:Phosphotyrosine