Purification and biochemical characterization of interchromatin granule clusters

Components of the pre-mrRNA splicing machinery are localized in interchroma tin granule clusters (IGCs) and perichromatin fibrils (PFs), Here we report the biochemical purification of IGCs, Approximately 75 enriched proteins w ere I,resent in the IGC fraction. Protein identification employing a novel mass spectrometry strategy and peptide microsequencing identified 33 known proteins, many of which have been linked to pre-mRNA splicing, as well as n umerous uncharacterized proteins. Thus fat; three new protein constituents of the IGCs have been identified. One of these, a 137 kDa protein, has a st riking sequence similarity over its entire length to UV-damaged DNA-binding protein, a protein associated with the hereditary disease xeroderma pigmen tosum group E, and to the 160 kDa subunit of cleavage polyadenylation speci ficity factor. Overall, these results provide a key framework that will ena ble the biological functions associated with the IGCs to be elucidated.