SRP samples nascent chains for the presenceof signal sequences by interacting with ribosomes at a discrete step during translation elongation

The signal recognition particle (SRP) binds to ribosomes that synthesize nascent chains bearing signal sequences and catalyzes their targeting to the endoplasmic reticulum membrane. In S. cerevisiae, a temperature-sensitive mutation in the SEC65 gene, encoding an SRP subunit, results in lowered levels of SRP. Growth and protein translocation defects induced by this mutation can be suppressed specifically by sublethal doses of cycloheximide but not anisomycin, each inhibitors of different steps of translation elongation. Cycloheximide also suppresses protein translocation defects caused by depletion of a different SRP subunit. We propose that reduced elongation rates in the presence of cycloheximide allow otherwise insufficient SRP to interact efficiently with ribosomes. These results are consistent with a sampling model in which SRP cycles on and off of translating ribosomes at specific steps during the elongation cycle to inspect all nascent chains for the presence of signal sequences.