LSm proteins form heptameric rings that bind to RNA via repeating motifs

Members of the LSm family of proteins share the Sm fold--a closed barrel comprising five anti-parallel beta strands with an alpha helix stacked on the top. The fold forms a subunit of hexameric or heptameric rings of approximately 7nm in diameter. Interactions between neighboring subunits center on an anti-parallel interaction of the fourth and fifth beta strands. In the lumen of the ring, the subunits have the same spacing as nucleotides in RNA, enabling the rings to bind to single-stranded RNA via a repeating motif. Eubacteria and archaea build homohexamers and homoheptamers, respectively, whereas eukaryotes use >18 LSm paralogs to build at least six different heteroheptameric rings. The four different rings in the nucleus that permanently bind small nuclear RNAs and function in pre-mRNA maturation are called Sm rings. The two different rings that transiently bind to RNAs and, thereby, assist in the degradation of mRNA in the cytoplasm and the maturation of a wide spectrum of RNAs in the nucleus are called LSm rings.     

SRC utilizes Cas to block gap junctional communication mediated by connexin43

The Src tyrosine kinase phosphorylates Cas (Crk-associated substrate) to confer anchorage independence and invasive growth potential to transformed cells. Gap junctional communication is often lower between aggressive tumor cells compared with normal or benign precursors. The gap junction protein connexin43 (Cx43) is a tumor suppressor that can inhibit tumor cell growth. Src can phosphorylate Cx43 to block gap junctional communication between transformed cells. However, mechanisms by which this event actually closes intercellular channels have not been clearly defined. Here, we report that Src and Cas associate with each other at intercellular junctions. In addition, Cas is required for Src to reduce dye transfer and electrical coupling between cells expressing Cx43. Thus, Src utilizes Cas to inhibit gap junctional communication mediated by Cx43. This finding introduces a novel role of the Cas focal adhesion linker protein in the gap junction complex. This observation may help explain how gap junctional communication can be suppressed between malignant and metastatic tumor cells.     

Spliceosome Sm proteins D1, D3, and B/B' are asymmetrically dimethylated at arginine residues in the nucleus

We report a novel modification of spliceosome proteins Sm D1, Sm D3, and Sm B/B′. L292 mouse fibroblasts were labeled in vivo with [³H]methionine. Sm D1, Sm D3, and Sm B/B′ were purified from either nuclear extracts, cytosolic extracts or a cytosolic 6S complex by immunoprecipitation of the Sm protein-containing complexes and then separation by electrophoresis on a polyacrylamide gel containing urea. The isolated Sm D1, Sm D3 or Sm B/B′ proteins were hydrolyzed to amino acids and the products were analyzed by high-resolution cation exchange chromatography. Sm D1, Sm D3, and Sm B/B′ isolated from nuclear fractions were all found to contain ω-N^G-monomethylarginine and symmetric ω-N^G,N^G′-dimethylarginine, modifications that have been previously described. In addition, Sm D1, Sm D3, and Sm B/B′ were also found to contain asymmetric ω-N^G,N^G-dimethylarginine in these nuclear fractions. Analysis of Sm B/B′ from cytosolic fractions and Sm B/B′ and Sm D1 from cytosolic 6S complexes showed only the presence of ω-N^G-monomethylarginine and symmetric ω-N^G,N^G′-dimethylarginine. These results indicate that Sm D1, Sm D3, and Sm B/B′ are asymmetrically dimethylated and that these modified proteins are located in the nucleus. In reactions in which Sm D1 or Sm D3 was methylated in vitro with a hemagglutinin-tagged PRMT5 purified from HeLa cells, we detected both symmetric ω-N^G,N^G′-dimethylarginine and asymmetric ω-N^G,N^G-dimethylarginine when reactions were done in a Tris/HCl buffer, but only detected symmetric ω-N^G,N^G′-dimethylarginine when a sodium phosphate buffer was used. These results suggest that the activity responsible for the formation of asymmetric dimethylated arginine residues in Sm proteins is either PRMT5 or a protein associated with it in the immunoprecipitated complex.