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306 Cell-Penetrating Peptides: Processes and Applications
14.4.1.2 Signal Anchors
Polytopic (multispanning) membrane proteins often do not have cleavable signal
peptides, although their most N-terminal transmembrane segment is thought to act
as an uncleavable signal peptide (see below). 8,9 However, some polytopic membrane
proteins are exceptional in that their most N-terminal polar segment translocates
across the membrane (the N-tail phenomenon). 160,161 Such an N-tail can be as long
as 100 residues. The mechanism of N-tail translocation is not well understood but
the effects of neighboring transmembrane segments have been observed. 162,163
In an in vitro system, SecA, SecB, and electrochemical potential were necessary
for a long N-tail translocation. 164 Although it is generally established that signal
peptides have sufficient information for their targeting to the membrane and their
translocation, there are some reports describing the contribution of other parts; for
example, human UDP-glucuronosyltransferase 1A6 has an internal signal sequence
that can direct the protein to the ER even when its N-terminal signal sequence is
deleted. 165 Prion protein also has a second ER targeting signal at its C terminus. 166
Another recent surprise was that a signal peptide can target to ER and to mitochondria.
167 In this case, an endoprotease cleavage of its N terminus can activate a cryptic
mitochondrial targeting signal. Interestingly, the topologies of proteins targeted to
each site were different, suggesting the difference of their membrane assembly
mechanisms.
14.4.2 SIGNAL PEPTIDASES
Signal peptidases are membrane-bound proteases that specifically remove signal
peptides from preproteins after their translocation. In eubacteria, they are classified
into two (or three) types: type I, type II, (and type III) signal peptidases. 168
14.4.2.1 Distribution of Type I Signal Peptidases
Type I signal (or leader) peptidases cleave most signal peptides; their homologs exist
in the ER, mitochondrial inner membrane, and chloroplast thylakoid membrane in
eukaryotes, as well as the plasma membrane of archaea. 168,169 Of these, they are
further divided into two types: the P (prokaryotic)-type for eubacterial, mitochondrial,
and chloroplastic peptidases and the ER-type for eukaryotic and archaeal
peptidases. 170 The P-type signal peptidases employ a serine–lysine catalytic dyad
mechanism instead of the standard serine–histidine–aspartic acid catalytic triad
mechanism of serine proteases. 168 In the ER-type, the lysine of the catalytic dyad is
hisidine. 169
Although most components for protein translocation are contained for only one
copy in a genome, some bacteria contain multiple paralogous copies of type I signal
peptidases. For example, seven copies were found in B. subtilis, two of which are
encoded in plasmids. Surprisingly, one of the five genes encoded in the chromosome,
SipW, seems to belong to the ER-type. 170 In addition, it was found that only two of
the five genes are of major functional importance and that their difference cannot
be explained from the structure of their catalytic site. 171