Component of the SRP (signal recognition particle) receptor (SR). Ensures, in conjunction with the signal recognition particle, the correct targeting of the nascent secretory proteins to the endoplasmic reticulum membrane system. GTP hydrolysis may enhance the fidelity of and provide unidirectionality to the targeting reaction. It is important but not essential for cell growth. May be directly involved in mitochondrial protein import. Heterodimer of an alpha and a beta chain. Thought to be anchored in the membrane through an interaction with SRP102/SR-beta, which contains a bona fide transmembrane domain. The SRX domain is sufficient for interaction with GTP-bound SRP102/SR-beta. The NG domain, also named G domain, is a special guanosine triphosphatase (GTPase) domain, which forms a guanosine 5'-triphosphate (GTP)-dependent complex with a homologous NG domain in the signal recognition particle (SRP) complex subunit SRP54 (By similarity). The two NG domains undergo cooperative rearrangements upon their assembly, which culminate in the reciprocal activation of the GTPase activity of one another (By similarity). GTPase induced rearrangement of SR drives SRP-mediated cotranslational protein translocation into the ER (By similarity). Present with 2000 molecules/cell in log phase SD medium. Belongs to the GTP-binding SRP family.