Interaction of intermediate filaments with ribosomes in vitro

Cytoplasmic intermediate filaments (cIFs) should be capable of loosely interacting with negatively charged, macromolecular assemblies, given that the net positive charge of the N-terminal head domains exposed on the surface of the cIFs is not fully neutralized by the neighboring C-terminal tail domains and the filament body proper. In order to substantiate this notion, cIFs reconstituted in vitro from various type III IF proteins and obtained as constituents of whole cell mount preparations were allowed to react at physiological ionic strength with 40S and 60S ribosomal subunits as well as 80S run-off ribosomes of mammalian origin. Electron micrographs of the reaction products show colocalization of all kinds of ribosomal particles with both the reconstituted and the natural cIFs, including the cytokeratin filaments of whole cell mount preparations of epithelial cells. The ribosome-cIF complexes are sensitive to ionic strength higher than physiological as well as to mild treatment with pancreatic RNase A, suggesting the direct involvement of the RNA moieties of the ribosomal particles in the binding reaction via the engagement of ionic bonds. This weak, electrostatic interaction potential of the cIF complement of the cytoskeleton might be exploited by the eukaryotic cell for the storage and distribution in the cytoplasm of non-polyribosomal ribonucleoprotein particles of the protein-synthesizing machinery and possibly also of negatively charged membrane vesicles.