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Protein transport events occurring at the endoplasmic reticulum
(ER) of eukaryotic cells and the cytoplasmic membrane of
prokaryotic organisms share many similarities. Resident proteins of
both membranes span the lipid bilayer once or several times by
a-helical stretches and their integration is usually mediated by
uncleaved signal-anchor sequences. Proteins that are translocated
across either membrane, collectively also termed secretory
proteins, harbour cleavable N-terminal signal sequences.
Prokaryotic and eukaryotic signal sequences have the same modular
structure and are functionally exchangeable. Integration of
membrane proteins and translocation of secretory proteins basically
occur at the same sites (pores) within each membrane. In both types
of membranes, these pores are c- posed of homologous components
forming the Sec translocons. Parts of the Sec trans- cons are found
populated by ribosomes, the membrane-bound ribosomes. Bacterial m-
brane and eukaryotic secretory proteins are targeted to the Sec
translocons by the same molecular mechanism involving signal
recognition particle (SRP) and its receptor (SRP - ceptor, SR).
Structure and assembly of the SRP The functional core of SRP The
functional core of this ribonucleoprotein complex consists of the
signal sequence binding subunit (SRP54 in eukaryotes and Ffh in
prokaryotes) and the SRP RNA molecule (see Fig. 1). This core is
conserved in all organisms, with the intriguing exception of
chloroplasts, where the SRP lacks the RNA subunit.
Protein transport events occurring at the endoplasmic reticulum
(ER) of eukaryotic cells and the cytoplasmic membrane of
prokaryotic organisms share many similarities. Resident proteins of
both membranes span the lipid bilayer once or several times by
a-helical stretches and their integration is usually mediated by
uncleaved signal-anchor sequences. Proteins that are translocated
across either membrane, collectively also termed secretory
proteins, harbour cleavable N-terminal signal sequences.
Prokaryotic and eukaryotic signal sequences have the same modular
structure and are functionally exchangeable. Integration of
membrane proteins and translocation of secretory proteins basically
occur at the same sites (pores) within each membrane. In both types
of membranes, these pores are c- posed of homologous components
forming the Sec translocons. Parts of the Sec trans- cons are found
populated by ribosomes, the membrane-bound ribosomes. Bacterial m-
brane and eukaryotic secretory proteins are targeted to the Sec
translocons by the same molecular mechanism involving signal
recognition particle (SRP) and its receptor (SRP - ceptor, SR).
Structure and assembly of the SRP The functional core of SRP The
functional core of this ribonucleoprotein complex consists of the
signal sequence binding subunit (SRP54 in eukaryotes and Ffh in
prokaryotes) and the SRP RNA molecule (see Fig. 1). This core is
conserved in all organisms, with the intriguing exception of
chloroplasts, where the SRP lacks the RNA subunit.
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