ELECTRON-MICROSCOPY OF ASSEMBLY INTERMEDIATES OF THE SNRNP CORE - MORPHOLOGICAL SIMILARITIES BETWEEN THE RNA-FREE (EFG) PROTEIN HETEROMER AND THE INTACT SNRNP CORE

All four spliceosomal small nuclear ribonucleoproteins (snRNPs) U1, U2 , U4/U6 and U5 contain a common structural element called the snRNP co re. This core is assembled from the common snRNP proteins and the smal l nuclear RNA (snRNA). We have used electron microscopy to study the s tructure of two intermediates of the snRNP core assembly pathway: (1) the (E.F.G) protein complex, which contains only the smallest common p roteins E, F and G; and (2) the subscore of U5 snRNP, in which the U5 RNA and the common proteins DI and D2 are bound to the (E.F.G) protein complex. The general structure of the subscore was found to resemble that of the complete snRNP core, which contains the components of the subscore plus the common proteins B/B' and D3. Both the complete snRNP core and subscore particles are globular, with diameters of 7 to 8 nm . They show a characteristic accumulation of stain at the centre. Howe ver, some subscore images showed nicked outlines not seen with the com plete snRNP cores. The (E.F.G) protein complex appeared as a ring, wit h an outer diameter of about 7 nm and a central hole 2 nm across. The molecular dimensions of the E, F and G proteins imply that the thickne ss of the (E.F.G) ring structure is only about 2 nm. Comparison of the (E.F.G) structure complex with the snRNP core and subcore structures implicates that a flat side of the ring-shaped (E.F.G) complex provide s the assembly site(s) for the other components of the snRNP during co re assembly: first for the D1 and D2 proteins (and probably the snRNA) during subscore formation, and then for the B/B' and D3 proteins in t he completion of the snRNP core particle. (C) 1997 Academic Press Limi ted