Sorting of Eukaryotic Proteins

Most of the arguments described in the sections on bacterial signal peptides and membrane proteins seem to be valid for the eukaryotic systems, as well as the translocation phenomena across the ER membrane (Sakaguchi, 1997). They seem to be also true for the translocation system across the mitochondrial inner membrane protein into the intermembrane space and the system across the thylakoid membrane in chloroplasts. Although the TAT-dependent pathway has not been found in the ER, it exists on the thylakoid membrane (and possibly on the inner membrane of mitochondria). 

The architecture of signal peptides including the ( — 3, —1) rule also holds in eukaryotes, although there are small differences of preferred residues (von Heijne, 1986b Nielsen et al., 1997). 

As described in Section II,C,2, some differences exist between the bacterial and eukaryotic systems on the multispanning membrane assembly (Gafvelin et al., 1997) however, they also have many points in common the multispanning membrane proteins are likely to be co-translationally integrated (Ulbrandt et al., 1997), and both systems use homologous translocon channels, which play an important role for the topogenesis of these multispanning membrane proteins (Prinz et al., 1998). 

Of the extremely diverse examples of protein modifications observed in eukaryotic cells, the modifications by lipid (and glycolipid) molecules are of special interest because lipid-attached proteins can be anchored at the membrane, although all of these proteins are not always anchored. So far, three groups of membrane anchoring proteins have been noted (Fig. 5). 

Some long-chain fatty acids are covalently linked to proteins by acylation. Of these, two types are observed rather frequently myristoylation and palmitoylation (Grand, 1989). 

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