Protein Glycosylation: Implications for In Vivo Functions and Therapeutic Applications - Advances in Biochemical Engineering Biotechnology - page 156

of precursor proteins across ER membranes occurs most likely through a pro-
tein-conducting channel [11]. The signal recognition particle (SRP) and nas-
cent-polypeptide-associated complex (NAC), a nonribosomal factor, together
provide for fidelity in protein targeting to ER and serve as a versatile targeting
chaperone team [12, 13]. Details of the mechanism of protein translocation
across membrane are beyond the scope of this review and are not discussed.
However readers are advised to see the current literature [14, 15].
2.2
Early Modifications of Protein
The cleavage of the signal peptide by signal peptidase occurs rapidly soon after
the translocation of protein across the membrane of rough ER. Prokaryotic
signal peptides can be released by eukaryotic signal peptidases and vice versa .
Protein folding is facilitated by the resident proteins of ER; some of them are
enzymes responsible for disulfide bond formation, isomerization of peptide
bonds and glycosylation. Disulfide bond formation is a major rate-limiting step
in protein folding,which involves the enzyme protein disulfide isomerase (PDI).
The PDI essentially unscrambles intermediates in the protein folding pathway
with non-native disulfide bonds [16]. Similarly, cis-trans isomerization of pro-
lylpeptide bonds is catalyzed by peptidyl-prolyl cis-trans isomerase [17]. Other
resident proteins are the classes of molecular chaperones, which appear to
stabilize protein folding intermediates, prevent competitive aggregation inter-
actions,or promote correct folding [18].They somehow recognize the kinetical-
ly trapped intermediate states of proteins, randomly disrupting and releasing
them in less folded states [19]. Their interiors provide a sticky hydrophobic
surface that competes with intrachain hydrophobic collapse and helps to pull
apart an incorrectly folded protein, allowing opportunities to find pathways
leading to the stable native state. Chaperones appear to act sequentially in pro-
tein folding pathways.
That folding begins cotranslationally is known for some proteins including
IgG, serum albumin and has been suggested for globin molecule [20]. Again, in
influenza hemagglutinin, disulfide bond formation and generation of con-
formational epitopes are cotranslational [21]. After initial modification in ER,
some proteins are retained (resident ER proteins) but the majority are exported
to final destinations within and outside the cell. Export from ER involves in-
corporation into transport vesicles that fuse with the next compartment along
the secretory pathway. The pathway consists of a series of membrane-bound
organelles between which proteins move in a vectorial manner. Specific signals
have been identified for the retention/retrieval of ER proteins. Retrieval of
soluble ER resident proteins from Golgi is mediated through recognition of amino
acid sequence 'Lys-Asp-Glu-Leu' by a specific receptor [22], while for transmem-
brane proteins, the intracytoplasmic dibasic motif plays a similar role [23].
Localization signals of Golgi GlycTs are more complex, and seem conforma-
tion dependent.Sequences in the cytoplasmic tails,transmembrane regions and