Cloned human snRNP proteins B and B' differ only in their carboxy-terminal part.

Abstract

Antibodies to the snRNP proteins B' and B are frequently observed in autoimmune diseases. We analyzed different types of cDNAs encoding these proteins. One type of cDNA encoded a protein whose predicted mol. wt is 24.6 kd, whereas another type encoded a protein with a predicted mol wt of 23.7 kd. When translated in vitro from cDNA transcripts, the apparent mol. wts of these proteins on SDS‐polyacrylamide gel were 29.5 and 28.5 kd respectively. The main difference between these two types of cDNAs proved to be the presence of an additional sequence of 146 nucleotides in the 3′ part of the open reading frame of the clone encoding the shorter protein. This insert contains a termination codon in frame, at 9 nucleotides downstream from the 5′ end of the insert. The additional sequence revealed at the 3′ end a consensus sequence of vertebrates for intron‐exon junctions. We demonstrated the presence of mRNAs corresponding with both types of cDNA in human cells. We hypothesize that the B' and B protein are derived from one pre‐mRNA by alternative splicing, and show that they differ only at the carboxy terminus, where a proline rich motive is repeated once more in B'. A comparison with amino acid sequences of other cloned snRNP proteins is included.

WW domain-mediated interactions reveal a spliceosome-associated protein that binds a third class of proline-rich motif: The proline glycine and methionine-rich motif

“…2B) suggested that FBP21 also may interact with U1C. The PGM motif has been described in SmB (and SmBЈ), U1C, and U1A proteins (28,29) and is also present in the SF1͞mBBP splice variant, which was isolated in our expression library screen and was described by Arning et al (30). The PGM domain is most striking in SmBЈ, where it is repeated three times (28) (Fig.…”

“…Eight positive plaques were purified, representing four different proteins. The snRNP core protein SmB (28,29) accounted for half of the interacting plaques. Another splicing factor, the SF1-HL1 splice variant of SF1͞mBBP (30), also was identified.…”

Protein Stoichiometry of a Multiprotein Complex, the Human Spliceosomal U1 Small Nuclear Ribonucleoprotein

“…The resulting peptides were labeled with [ 12 C]propionyl-N-oxysuccinimide. Known amounts of synthetic peptides, labeled with the corresponding 13 C isotope-coded amine-specific tag, were added (*Reference). MALDI-TOF mass spectrometry was used to absolutely quantify the proteins in the U1 spliceosomal snRNP.…”

“…Furthermore, this technique has also been used to study the (relative) dynamics of protein complexes in different states by the incorporation of stable isotopes (1, 2), a method widely used in quantitative proteomics. Labeling with stable isotopes can be achieved either by chemical modification of side-chain amino groups with a reagent containing heavier isotopes ( 2 H, 13 C, 15 N) (3,4) or by incorporation of isotope-coded amino acids from the growth medium (5,6). To understand the function of individual proteins in a complex, it is necessary to analyze the interacting proteins in a quantitative way, which reveals whether a certain protein is present in defined stoichiometric amounts or forms only a weak substoichiometric interaction.…”

“…Some arginine residues of SmB and SmD3 have symmetrical dimethyl modifications (12). In humans, some tissues, as well as HeLa cells, contain a second SmB protein called SmBЈ, which is a splicing variant in which the last two amino acid residues are replaced by a new sequence of eleven residues (13). When bound to the snRNA, the Sm proteins form a very stable core RNP structure.…”