The properties of the RNA-binding protein NF90 are considerably modulated by complex formation with NF45.

Nuclear factor 90 (NF90) is an RNA-binding protein (RBP) that regulates post-transcriptionally the expression of various mRNAs. NF90 was recently shown to be capable of discriminating between different RNA substrates. This is mediated by an adaptive and co-operative interplay between three RNA-binding motifs (RBMs) in the protein's C-terminus. In many cell types, NF90 exists predominantly in a complex with NF45. Here, we compared the RNA-binding properties of the purified NF90 monomer and the NF90-NF45 heterodimer by biophysical and biochemical means, and demonstrate that the interaction with NF45 considerably affects the characteristics of NF90. Along with a thermodynamic stabilization, complex formation substantially improves the RNA-binding capacity of NF90 by modulating its binding mode and by enhancing its affinity for single- and double-stranded RNA substrates. Our data suggest that features of both the N- and C-termini of NF90 participate in the heterodimerization with NF45 and that the formation of NF90-NF45 changes the conformation of NF90's RBMs to a status in which the co-operative interplay of the RBMs is optimal. NF45 is considered to act as a conformational scaffold for NF90's RBMs, which alters the RNA-binding specificity of NF90. Accordingly, the monomeric NF90 and the NF90-NF45 heterodimer may exert different functions in the cell.