BACKGROUND: Mutations in the ribosomal protein S19 gene (RPS19) have been found 
in 25% of patients with Diamond-Blackfan anemia, a rare syndrome of congenital 
bone marrow failure characterized by erythroblastopenia and various 
malformations. Mechanistic understanding of the role of RPS19 in normal 
erythropoiesis and in the Diamond-Blackfan anemia defect is still poor. However, 
defective ribosome biogenesis and, in particular, impaired 18S ribosomal RNA 
maturation have been documented in association with various identified RPS19 
mutations. Recently, new genes, all encoding ribosomal proteins, have been found 
to be mutated in Diamond-Blackfan anemia, adding further support to the concept 
that ribosome biogenesis plays an important role in regulating erythropoiesis. 
We previously showed variability in the levels of expression and subcellular 
localization of a subset of RPS19 mutant proteins.
DESIGN AND METHODS: To define the mechanistic basis for this variability better, 
we studied a large number of mutant proteins and characterized both RPS19 
expression level using a specific antibody against RPS19 and RPS19 subcellular 
localization after transfection of Cos-7 cells with various green fluorescent 
protein-RPS19 mutants. To investigate the role of the proteasome in RPS19 
degradation, we examined the effect of various proteasome inhibitors, namely 
lactacystin, MG132, and bortezomib on RPS19 expression and subcellular 
localization
RESULTS: We found two distinct classes of RPS19 protein defects in 
Diamond-Blackfan anemia based on the stability of the mutant proteins: (i) 
slightly decreased to normal levels of expression and normal nucleolar 
localization and (ii) markedly deficient expression and failure to localize to 
the nucleolus. All the proteasome inhibitors tested were able to restore the 
expression levels and normal subcellular localization of several unstable mutant 
proteins.
CONCLUSIONS: Our findings demonstrate an important role for the proteasomal 
degradation pathway in regulating the expression levels and nucleolar 
localization of certain mutant RPS19 proteins in Diamond-Blackfan anemia.