Transport of Proteins and Signal Recognition
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We provide support for the belief that the mRNA for a given protein contains the entire information for its final localization in a cell or its secretion out of a cell. If mRNA coding for carp preproinsulin is injected into Xenopus cocytes, proinsulin is found in the medium. Similarly, if the 26S-RNA coding for the structural proteins of the Semliki-Forest-Virus is injected, the envelope proteins are found on the outer surface of the oocytes. Transport and processing of the membrane proteins are not dependent on glycosylation. However, if plant storage globulin mRNAs are injected into oocytes, the translation products are found to be secreted rather than stored. Obviously, in this case the destination of the proteins is determined in part by the cellular apparatus.
Carp preproinsulin, like most other secretory proteins, contains a N-terminal, cleavable signal peptide. Recombinant plasmids were constructed which contained the complete coding sequence of carp preproinsulin placed into the coding region of prokaryotic genes so that fused translation products are expected. In many cases, the eucaryotic signal peptide was found to transport the insulin antigen out of E.coli cells even if its sequence was internal. Cleavage of the signal peptide is apparently correct since a proinsulin-like material is found in the periplasm. The results support our hypothesis according to which the first stretch of hydrophobic amino acids, following an unfolded part of the polypeptide chain or a complete folding domain with a hydrophilic surface, operates as signal for translocation.
KeywordsSignal Peptide Xenopus Oocyte Envelope Protein Signal Recognition Translation Product
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