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.