Component of the SRP (signal recognition particle) receptor (SR) (By similarity). Ensures, in conjunction with the signal recognition particle, the correct targeting of the nascent secretory proteins to the endoplasmic reticulum membrane system (By similarity). Forms a guanosine 5'-triphosphate (GTP)-dependent complex with the SRP subunit srp54 (By similarity). SRP receptor compaction and GTPase rearrangement drive SRP-mediated cotranslational protein translocation into the ER (By similarity). Heterodimer of an alpha and a beta chain. Thought to be anchored in the membrane through an interaction with srprb, which contains a bona fide transmembrane domain. 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). Belongs to the GTP-binding SRP family.