SR protein

SR proteins are Serine/Arginine-rich proteins which are involved in regulating and selecting splice sites in eukaryotic mRNA. Alternative splicing requires SR proteins to select which alternative splice sites should be used.

Most evidences indicated that SR proteins are encoded by essential genes. Recently a RNAi study in C. elegans was not able to confirm the essentiality. However when several SR mRNAs were targeted by RNAi simultaneously, lethality was observed.

Location and translocation
SR proteins are localized in speckle domain in the interphase nucleus. Some SR proteins travels between nucleus and cytoplasm. Subcellular localization of SR proteins are affected by phosphorylation. Recent studies (in 2006) suggest fundamental differences in the regulation of the mobility of plant (ATP dependent) and animal (ATP independent) SR splicing factors.

Structure
All SR proteins have one or two N-terminal RNA binding domain and C-terminal protein interaction domain. The latter which is rich in serine and arginine is called SR domain. Serines in SR domain may be extensively phosphorylated. This is a method by which cell can regulate the localization of the proteins. One SR domain can interact with another SR domain. SR domains and RNA recognition motifs (RRM) are modular. SR domain can be seen in proteins other that SR family.

Function
SR proteins have been shown to have roles in alternative and constitutive splicing inadition to roles in translation.

Exon dependent roles (splicing)

 * Recruitment of U1 and U2AF
 * Splicing regulatory functions

Exon independent roles (splicing)

 * SR protein may have a role in recruiting U4U5U6 tri-complex.