SR (ser/arg) proteins have been shown to play roles
in numerous aspects of pre-mRNA splicing, including modulation
of alternative splicing, commitment of substrates to the
splicing pathway, and splice site communication. The last
of these, splice site communication, is particularly relevant
to trans-splicing in which the 5′ and 3′
exons originate on separate molecules. The participation
of SR proteins in naturally occurring, spliced leader RNA-dependent
trans-splicing has not been examined. Here, we
have isolated SR proteins from an organism that performs
both trans- and cis-splicing, the nematode
Ascaris lumbricoides. To examine their activity
in in vitro splicing reactions, we have also developed
and characterized an SR protein-depleted whole-cell extract.
When tested in this extract, the nematode SR proteins are
required for both trans- and cis-splicing.
In addition, the state of phosphorylation of the nematode
SR proteins is critical to their activity in vitro. Interestingly,
mammalian (HeLa) and A. lumbricoides SR proteins
exhibit equivalent activities in cis-splicing,
while the nematode SR proteins are much more active in
trans-splicing. Thus, it appears that SR proteins
purified from an organism that naturally trans-splices
its pre-mRNAs promote this reaction to a greater extent
than do their mammalian counterparts.