The signal sequence of nascent preprolactin interacts with the 54-kD protein of the signal recognition particle (SRP54). To identify the domain or site on SRP54 that interacts with the signal sequence we used a photocross-linking approach followed by limited proteolysis and immunoprecipitation using anti-peptide antibodies specific for defined regions of SRP54. We found that the previously identified methionine-rich RNA-binding domain of SRP54 (SRP54M domain) also interacts with the signal sequence. The smallest fragment that was found to be crosslinked to the signal sequence comprised the COOH-terminal 6-kD segment of the SRP54M domain. No cross-link to the putative GTP-binding domain of SRP54 (SRP54G domain) was found. Proteolytic cleavage between the SRP54M domain and SRP54G domain did not impair the subsequent interaction between the signal sequence and the SRP54M domain. Our results show that both the RNA binding and signal sequence binding functions of SRP54 are performed by the SRP54M domain.

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