This report describes a novel RNA-binding protein,
SECp43, that associates specifically with mammalian selenocysteine
tRNA (tRNASec). SECp43, identified from a degenerate
PCR screen, is a highly conserved protein with two ribonucleoprotein-binding
domains and a polar/acidic carboxy terminus. The protein
and corresponding mRNA are generally expressed in rat tissues
and mammalian cell lines. To gain insight into the biological
role of SECp43, affinity-purified antibody was employed
to identify its molecular partners. Surprisingly, the application
of native HeLa cell extracts to a SECp43 antibody column
results in the purification of a 90-nt RNA species identified
by direct sequencing and Northern blot analysis as tRNASec.
The purification of tRNASec by the antibody
column is striking, based on the low abundance of this
tRNA species. Using recombinant SECp43 as a probe for interacting
protein partners, we also identify a 48-kDa interacting
protein, which is a possible component of the mammalian
selenocysteine insertion (SECIS) pathway. To our knowledge,
SECp43 is the first cloned protein demonstrated to associate
specifically with eukaryotic tRNASec.