The Signal Recognition Particle and Translocation
In some types of eukaryotic cells, most of the proteins translocated into or across membranes utilize the signal recognition particles. These are small ribonucleoprotein particles containing a 300 nucleotide RNA molecule and five proteins, ranging in size from 14 KDa to 72 KDa. As the elongating signal peptide protrudes from the ribosome, the signal recognition particles bind and arrest translation. After binding of the signal recognition particle to the endoplasmic reticulum, translation resumes and the protein is translocated across the membrane during its synthesis.
The generality of this process is not known, and some eukaryotic cells seem not to arrest translation. Controversy has waxed hot on the matter of whether bacteria also utilize the same sort of pathway. They contain a small ribonucleoprotein particle containing an RNA that sediments at 4.5S. This possesses significant homology to the eukaryotic signal recognition particle, but its role in protein secretion is not yet clear.
Can the roles of the signal recognition particle components be iden-tified? Genetics frequently can be utilized in simpler organisms to obtain mutants unable to perform certain reactions. The complexity of eukaryotic systems blocks the use of mutants for functional dissection of the signal recognition particle. Instead, a biochemical approach had to be used.
There are two requirements for a biochemical dissection. First, it must be possible to assay for each of the steps in the process. This is possible. Binding of signal recognition particles can easily be measured because after binding they cosediment with the translating ribosomes. By using homogenous messenger, translation arrest can be seen by the failure of the proteins to be completed and by the accumulation of short incomplete proteins. Translocation into membranes can be quantitated by adding membrane vesicles to a translation mixture. Translocation of the protein into the vesicles leaves them resistant to protease digestion.
The second requirement is the ability to take the signal recognition particle apart and then to reassemble it. When the particle had been dissociated and the various protein components were separated, they were individually inactivated by reaction with N-ethylmaleimide. Par-ticles were reassembled containing one reacted component and the remainder unreacted. By this means, the proteins responsible for signal recognition, translation arrest, and translocation were identified. In contrast to many systems, each of these functions was determined by a separate component of the particle. Many times, functions are shared.