Regulated gene expression at the post-transcriptional level in higher eukaryotes 
is based on a network of interactions among RNA-binding proteins (RBPs) 
operating within multifactorial ribonucleoprotein (RNP) complexes, notably 
heterogeneous nuclear ribonucleoprotein (hnRNP) and mRNP complexes. We are 
interested in interactions involving hnRNP proteins participating in several 
steps of mRNA processing (mainly pre-mRNA splicing) and HuR with an established 
role in stability/translation of associated mRNAs. hnRNP and HuR proteins have a 
major nucleoplasmic localization and ability to shuttle between nucleus and 
cytoplasm. We report here on interactions between hnRNP and HuR proteins that 
were identified in the context of isolated hnRNP and mRNP complexes. This was 
done by the application of immunoprecipitation and pull-down assays on different 
sub-cellular fractions prepared from cells of human and mouse origin, as well as 
in vivo localization studies. A range of specific associations of HuR with the 
shuttling hnRNP A1 and A3 and the non-shuttling hnRNP C1/C2 was identified and 
ascribed discrete properties with respect to stability to RNase A and increasing 
salt, as well as to cellular distribution. The likelihood of a biological 
relevance of these associations was tested under heat shock conditions in 
growing cells, which appeared to affect both the sub-nuclear distribution and 
interaction of HuR with hnRNPs. The establishment of an extensive association of 
HuR with hnRNP components of nuclear hnRNP/mRNP and cytoplasmic mRNP complexes 
supports its broader participation in mRNA processing events than initially 
anticipated.