Question

The role of the signal recognition particle (SRP) in sorting proteins that contain an ER signal sequence is

• A. the SRP must be associated with a cytosolic ribosome before the ribosome can attach to the ER membrane and initiate translation of an mRNA encoding a protein with an ER signal
• B. the SRP binds the signal sequence in the cytosol after synthesis of the protein has begun on a ribosome, and escorts the ribosome/mRNA complex to the ER membrane
• C. the SRP is an integral membrane protein in the ER that binds the signal sequence and guides the ribosome translating the protein to a translocation channel
• D. the SRP cleaves the signal sequence from the end of the protein after translocation into the ER, releasing the protein into the ER lumen
• E. the SRP is the largest subunit of the translocation channel that binds and anchors the signal sequence as the protein is translocated through the ER membrane

Dr. Guenter Blobel proposed his “signal hypothesis” in 1971, based on his experiments with in vitro (in the test tube) synthesis of an antibody. In living cells, the large and small subunits of the antibody he was studying, which is secreted from cells, would contain

• A. a single internal start-transfer signal sequence
• B. an N-terminal signal sequence that is cleaved, followed by an internal stop-transfer sequence
• C. an N-terminal signal sequence that is not cleaved from the mature protein
• D. a C-terminal KDEL signal sequence
• E. an N-terminal cleaved signal sequence

Answer 1

1) signal recognition particle is found in the cytosol. Ribosome in the cytosol translates the N terminal signal sequence, SRP can bind to the signal sequence, then translation pauses, SRP then replaces the bound GDP with GTP and then binds to the SRP receptor in the ER membrane, then hydrolyzes the GTP and loads the mRNA-ribosome, signal sequence complex into a translocon in the ER membrane and translation resumes so the answer is

B. the SRP binds the signal sequence in the cytosol after synthesis of the protein has begun on a ribosome and escorts the ribosome/mRNA complex to the ER membrane.

2) if the protein is soluble the N terminal signal sequence is cleaved in ER and the protein is translated to the Er lumen, from where it is packed in vesicles to Golgi-apparatus and from there protein is transported in vesicles which fuse with the plasma membrane and release the antibodies so the answer is

E. an N-terminal cleaved signal sequence