SR alpha

The receptor for Signal Recognition Particle (SRP) is the site on the endoplasmic reticulum that ribosomes translating secreted and integral membrane proteins are initially targeted to. Once the ribosome-SRP complex arrives at the SRP receptor the protein being translated is transferred to the translocation complex (Sec61) in the ER membrane. The SRP receptor is composed of two subunits SRalpha and SRbeta. The SRalpha subunit is a translocation GTPase peripherally bound to the endoplasmic reticulum by its interaction with SRbeta. SRalpha also binds to the GTPase of SRP (SRP54) and these two proteins appear to function as each others GTPase activating proteins (GAPs). Hydrolysis of GTP by SRalpha and SRP54 is thought to be involved in transfer of the nascent protein to the Sec61 complex in the ER. SRalpha has an apparent molecular weight of 72 kDa. SRbeta is a Type I transmembrane protein that spans the membrane once and contains Ras type GTPase domain. The function of the GTPase in SRbeta is unknown. The membrane spanning domain is at the amino-terminus of SRbeta. The GTPase domain encompasses three quarters of the protein and is carboxyl- of the transmembrane region. SRalpha binds to the GTPase domain of SRbeta. Heterodimerization of SRalpha and SRbeta masks the carboxyl-terminal epitope of SRbeta.