Fluorescence studies on the role of tryptophan in heterogeneous nuclear ribonucleoprotein particles of HeLa cells.

The 40 S heterogeneous nuclear ribonucleoprotein (hnRNP) particles from HeLa cells reveal tryptophan fluorescence with a bi-exponential decay, indicating that only a few of the 'core' proteins contain tryptophan residues. The presence of tryptophan residues distinguishes hnRNP particles from nucleosomes, with which they otherwise share a number of properties. This difference, however, is not essential for protein-RNA binding, as the fluorescence decay remains unchanged when hnRNP particles are dissociated into protein and RNA. However, the Stern-Volmer quenching constant is doubled upon salt dissociation, i.e. tryptophan residues become more accessible to solvent. Thus tryptophan quenching is a useful parameter for monitoring protein-protein interactions in hnRNP particles.