U5 small nuclear ribonucleoprotein (
snRNP), purified from HeLa nuclear extracts (splicing extracts), shows a complex
protein composition. In addition to the
snRNP proteins B', B,
D, D', E, F, and G, which are present in each of the major snRNPs U1, U2, U4/U6, and U5,
U5 snRNP contains a number of unique
proteins characterized by apparent molecular masses of 40, 52, 100, 102, 116, and 200 (mostly a double band) kDa. The latter set of
proteins may be regarded as U5-specific for the following reasons. They are not only eluted specifically, together with
snRNP particles, from anti-2,2,7-trimethylguanosine immunoaffinity columns by
7-methylguanosine, they also cofractionate with
U5 snRNP during chromatography and, most importantly, in
glycerol gradient centrifugation. These
U5 snRNP particles show a high sedimentation constant of about 20S. U5 snRNPs that lack the U5-specific
proteins are also found in nuclear extracts but have (in comparison) a lower sedimentation value of only 8-10S. Autoimmune sera from patients with
systemic lupus erythematosus were identified that, on immunoblots with purified
U5 snRNP proteins, reacted selectively with the 100- or 200-kDa
proteins. This indicates that at least the high molecular mass U5-specific
proteins are structurally distinct and not derived one from the other by proteolytic degradation. The existence of so many unique
proteins in the
U5 snRNP suggests that this
snRNP particle may exert its function during splicing mainly by virtue of its
protein components.