THE PLANT U1 SMALL NUCLEAR RIBONUCLEOPROTEIN PARTICLE 70K PROTEIN INTERACTS WITH 2 NOVEL SERINE ARGININE-RICH PROTEINS/

The U1 small nuclear ribonucleoprotein particle (U1 snRNP) 70K protein (U1-70K), one of the three U1 snRNP-specific proteins, is implicated in basic and alternative splicing of nuclear pre-mRNAs. We have used t he Arabidopsis U1-70K in the yeast two-hybrid system to isolate cDNAs encoding proteins that interact with it. This screening has resulted i n the isolation of two novel plant serine/arginine-rich (SR) proteins, SRZ-22 and SRZ-21 (SRZ proteins). Neither the N-terminal region nor t he arginine-rich C-terminal region of U1-70K alone interact with the S RZ proteins. The interaction of U1-70K with the SRZ proteins is confir med further in vitro using a blot overlay assay. The plant SRZ protein s are highly similar to each other and contain conserved modular domai ns unique to different groups of splicing factors in the SR family of proteins. SRZ proteins are similar to human 9G8 splicing factor becaus e they contain a zinc knuckle, precipitate with 65% ammonium sulfate, and cross-react with the 9G8 monoclonal antibody. However, unlike the 9G8 splicing factor, SRZ proteins contain a glycine hinge, a unique fe ature in other splicing factors (SC35 and ASF/SF2), located between th e RNA binding domain and the zinc knuckle. SRZ-22 and SRZ-21 are encod ed by two distinct genes and are expressed in all tissues tested with varied levels of expression. Our results suggest that the plant SRZ pr oteins represent a new group of SR proteins. The interaction of plant U1-70K with the SRZ proteins may account for some differences in pre-m RNA splicing between plants and animals.