Ultrastructural morphology of the hnRNP C protein tetramer.

The C protein tetramer is one of three heterotetramers which comprise the majority of the protein mass of mammalian 40S nuclear ribonucleoprotein particles (hnRNP particles). The events of RNA processing occur while the nascent transcripts are packaged in these structures. The C protein tetramer contains three monomers of C1 and one C2 monomer [i.e., (C1)3C2]. The tetramer's mass (129.2 kDa), approximate sedimentation coefficient (5.8S), and Stokes radius (6.2 nm) suggest that the tetramer may be either highly anisotropic or may possess an unusually large hydration shell in solution. The tetramer binds approximately 235 nucleotides of pre-mRNA. Electron microscopy of purified individual RNA-free C protein tetramers has revealed the overall morphology of this important pre-mRNA binding complex. In negatively stained preparations, the tetramer clearly displays a nonlinear, three- or four-lobed appearance with a diameter of 8.5 +/- 0.5 nm. A detailed comparison of the substructure seen in individual images suggests a tetrahedral arrangement of the four polypeptides. Rotary-shadowed images confirm the size of the tetramer observed in negatively stained preparations. This study provides the first demonstration of the overall arrangement of polypeptides in the C protein tetramer.