Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 221

Archaea and eukaryotes do not have SecB or SecA and use the SRP system to translocate all exported proteins. Although they lack SecA, they may have other systems that help direct already translated proteins to the translocon. The translocon itself was first discovered in eukaryotes and is composed of three proteins that form similar structures in all three kingdoms of life. The amino acid sequences of the SecY and SecE subunits are similar in all three kingdoms; only the sequence of the third subunit (SecG in bacteria) is very different in eukaryotes and archaea, where it may have different functions. While eukaryotes have other such channels, the translocase, which helps transported proteins to enter the endoplasmic reticulum of eukaryotic cells, is the one most similar to the SecYEG channel of bacteria.

The SRP system was also first described in eukaryotes, where it is much larger, consisting of a 300-nucleotide RNA and eight proteins, six in the SRP and two in the docking protein, called the SRP receptor. However, some of the proteins in eukaryotes are very similar to those in bacteria, such as the 54-kDa SRP protein in eukaryotes, which is similar to the Ffh protein in the SRP of bacteria. The SRP system of eukaryotes targets both membrane and presecretory proteins to the endoplasmic reticulum.