Family 3.A.5 - The Type II (General) Secretory Pathway Family
Family ID: 52652
Protein secretion in bacteria can be achieved by an ABC-type transport
system (the Type I protein secretion system, TC #3.A.1), the general
secretory pathway (the Sec or Type II protein secretion system
described here), and three additional types of systems (the Type
III, Type IV and Tat protein secretion systems, TC #3.A.6, 3.A.7
and 2.A.64, respectively). Protein complexes of the IISP family
are found universally in prokaryotes and eukaryotes. The translocase
in E. coliconsists of three integral inner membrane proteins,
SecYEG, and the cytoplasmic ATPase, SecA. SecA recruits SecYEG
complexes to form the active translocation channel. The active
assembly consists of a SecA homodimer and four SecYEG complexes.
Based on a 9 Å projection structure, the SecYEG complex
may exist both as an assembled tetrameric channel and as an unassembled
smaller unit, suggesting that transitions between the two states
occur during a functional cycle (Collinson et al., 2001).
SecY is a
10 TMS protein of about 450 amino acyl residues that is believed
to form the protein translocating channel. Two smaller integral
membrane proteins, SecE and SecG, each about 140 amino acyl residues
in length, are found complexed with SecY. Translocation is driven
by ATP hydrolysis catalyzed by the SecA ATPase constituent of
the translocase which associates tightly with SecY. Both SecY
and SecA directly contact the substrate protein. Although protein
export is driven by ATP hydrolysis, the pmf is stimulatory. Possibly
both energy sources are required for efficient translocation,
with each acting at different steps. Point mutations in SecY abolish
the pmf-dependence of the translocation process, but ATP hydrolysis
is essential under all conditions.
The SecY proteins
of archaea and the Sec61 proteins in the endoplasmic reticula
of Saccharomyces cerevisiaeand other eukaryotes show sequence
similarity to and are homologous to SecY of E. coliand other bacteria.
One of the proteins of the E. coli IISP main terminal branch (MTB)
complex (the secretin) is homologous to one of the constituents
of the Type III protein secretory system (TC #3.A.6), and one
constituent of the MTB (an ATPase) is homologous to an ATPase
(VirBII) of the Type IV SP complex (TC #3.A.7). Thus, they share
certain minimal structural and functional features. The Sec system
can both translocate proteins across the cytoplasmic membrane
and insert integral membrane proteins into it. The former proteins
but not the latter proteins possess N-terminal, cleavable, targeting
signal sequences that are required to direct the proteins to the
Sec complex.
The SecY-Sec61a
phylogenetic tree reveals ten clusters according to organismal
phylogeny as follows: (1) four clusters from Gram-negative bacteria
(proteobacteria, spirochetes, chlamydia and primitive bacteria),
(2) two clusers from Gram-positive bacteria (high and low G+C
organisms), (3) Mycoplasma,(4) cyanobacteria and eukaryotic chloroplasts,
(5) archaea, (6) eukaryotes.
In eukaryotes,
the heterotrimeric Sec61 protein complex in the endoplasmic reticulum
(ER) serves as the channel for protein transport by either a cotranstranslational
or posttranslational mechanism. In cotranslational export, directionality
is determined by binding of the translating ribosome to the Sec61
complex. The channels in the ribosome and membrane are aligned
so the luminal end of the channel is the only exit site available
to the elongating polypeptide chain. By contrast, in posttranslational
transport, the Sec61 complex associates with the tetrameric Sec62/63
complex, the resultant Sec complex binds the signal sequence of
the translocation substrate, and translocation is energized by
BiP (Kar2), a soluble, luminal Hsp70 ATPase that hydrolyzes ATP
to translocate polypeptides. Translocation requires that BiP interacts
with the Sec complex via a luminal domain of Sec63, the J domain.
BiP may "pull" the protein through the channel and/or
act as a "molecular ratchet", preventing backward movement.
While both mechanisms may be operative, the ratchet mechanism
is clearly operative under certain conditions (Matlack et al.,
1999; Misselwitz et al., 1998).
Considerable
evidence suggests that the ER translocon can function as a "retrotranslocon"
to transport improperly folded proteins from the lumen of the
ER, back into the cytoplasm where degradation occurs in proteosomes.
Thus, ER lumen proteins that are stalled at some point in their
folding/assembly, and possibly integral membrane proteins that
do not properly fold, may be recognized by specific chaparone
proteins and targeted for retrotranslocation (Johnson and Haigh,
2000). The process requires cytoplasmic proteins and ATP, but
the specific mechanism of energy coupling is not known. In one
case, that of the cholera toxin Al chain, protein disulfide isomerase
acts as a redox-dependent unfoldase, feeding the toxin into the
Sec61 complex for retrotranslocation (Tsai et al., 2001).
The Sec secretory
pathway functions in transport of proteins across the cytoplasmic
membrane. A distinct protein complex, termed the main terminal
branch (MTB) of the IISP, is responsible for exoprotein secretion
across the outer membrane of a wide variety of Gram-negative bacteria.
The MTB is complex, consisting of at least 14 proteins that somehow
function in the pmf-energized transport of exoproteins from the
periplasm across the outer membrane to the external milieu. The
best characterized MTB system is the pullulanase secretion system
of Klebsiella oxytoca,but several other MTB complexes have been
characterized. The actual integral outer membrane protein porin
of this system is the PulD secretin (the product of the pulDgene),
a member of the Secretin family (TC #1.B.22).
