The Role of Guanine Ribonucleotides in Protein Translocation Across the Mammalian Endoplasmic Reticulum: a Thesis

Publication Date

September 1989

Document Type

Doctoral Dissertation


Graduate School of Biomedical Sciences


Endoplasmic Reticulum; Guanine Nucleotides; Academic Dissertations; Dissertations, UMMS


The SRP and SRP receptor have long been recognized as essential components of the protein translocation machinery in higher eukaryotes. The biochemical studies discussed in this thesis demonstrate that the signal recognition particle (SRP) mediated transport of proteins across the mammalian endoplasmic reticulum requires the participation of guanine ribonucleotides, in a capacity distinct from their role in polypeptide elongation. The requirement for guanine ribonucleotides during translocation was detected by experimentally separating the synthesis and transport phases of the translocation reaction. Here, the initial targeting of ribosomes to the membrane required SRP and an SRP receptor, but not GTP. However, the insertion of the nascent chain into the membrane required the presence of both SRP and SRP receptor, as well as, GTP.

Further biochemical characterization of the initially targeted translocation intermediate demonstrated that SRP remains bound to targeted nascent signal sequences, unless GTP is present. The SRP-receptor catalyzed displacement of SRP from ribosomes was GTP-dependent both with intact membranes and with the purified SRP receptor preparations. GTP specific binding localized to the α subunit of the receptor by photoaffinity labeling and by probing nitrocellulose blots of the receptor with GTP. In addition, an analysis of the α subunit primary sequence revealed elements which are similar, yet not identical, to guanine ribonucleotide binding site consensus sequence elements. These results, taken together, indicate that the SRP receptor represents a novel class of GTP binding protein and is responsible for the guanine ribonucleotide mediated displacement of SRP from nascent signal sequences.

A more detailed biochemical investigation of the GTP hydrolysis cycle of the SRP receptor demonstrated that the affinity between SRP and the SRP receptor is substantially greater in the presence of bound GTP and that the subsequent hydrolysis of bound GTP by SRα is necessary to recycle SRP to the cytoplasm. Purified SRP receptor was shown to hydrolyze GTP slowly. However, the GTP hydrolysis rate was substantially increased when both the SRP receptor and SRP were present in equimolar quantity. SRP does not hydrolyze GTP under these assay conditions. Moreover, free SRP was found not to compete effectively with SRP-ribosome complexes for the receptor, implying that the conformation of SRP is altered upon binding to a signal sequence. This result suggests that the affinity between SRP and the SRP receptor may be exquisitely regulated in order to prevent futile GTP hydrolysis cycles from occurring in the absence of secretory protein synthesis.

Furthermore, the demonstration that the SRP receptor is a GTP binding protein provides fundamental insight into the mechanism of protein translocation. The displacement of SRP appears to be tightly coupled to the membrane insertion of nascent signal sequences. The membrane inserted intermediate in nascent chain translocation can be characterized by i) a resistance to extraction from the membrane with either EDTA or 0.5M KOAc; ii) an insensitivity to protease digestion, even after dissolution of the membrane with nonionic detergent. These results indicate that SRP displacement allows the nascent chain to interact with an additional membrane bound, protein component of the cellular translocation apparatus. Once in contact with this additional component, the nascent chain is shown to be capable to transverse the membrane bilayer in the absence of ribonucleotide hydrolysis or the continued elongation of the polypeptide. Thus, the results are incompatible with postulated mechanisms of protein translocation requiring that energy be derived from the continued elongation of the nascent polypeptide or from the direct interaction of a hydrophobic signal sequence with the lipid bilayer.


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