SecA: Difference between revisions

=Introduction=

=Structure=

[http://www.nature.com/nature/journal/v455/n7215/full/nature07335.html SecA] SecA consists of two RecA-like nucleotide-binding domains (NBD1 and NBD2), which bind the nucleotide between them, a polypeptide-cross-linking domain (PPXD), a helical scaffold domain (HSD) and a helical wing domain (HWD)<ref name=journal2>PMID:18923516</ref> Although several crystal structures of isolated SecA have been determined, the function of the different domains and the mechanism by which SecA moves polypeptides through the channel remain unknown. Disulphide cross-linking experiments suggest that SecA binds by its NBD1 domain to a non-translocating SecY copy, and moves the polypeptide chain through a neighbouring SecY molecule6. These and other experiments indicate that SecA functions as a monomer during translocation<ref name=journal2/>but the issue remains controversial.<ref name=journal2/>

Here we report crystal structures of SecA bound in an intermediate state of nucleotide hydrolysis to the SecY channel. The structures suggest mechanisms for how the channel is opened and prepared for the arrival of a translocation substrate, and how SecA moves polypeptides through the channel.  

==Structure Determination Of SecA-SecY Complexes==

'''For a figure of the SecA-SecY complex click here''' [http://www.nature.com/nature/journal/v455/n7215/fig_tab/nature07335_F1.html SecA-SecY Complex]

[http://journal.shouxi.net/qikan/article.php?id=418668 SecA] SecA interacts not only with the SecY<ref name=journal1/> channel  but also with acidic phospholipids (9-11) and with both the signal sequence and the mature part of a substrate protein<ref name=journal1/>. It also binds the chaperone SecB, which ushers some precursor proteins to SecA<ref name=journal1/>. When associated with the SecY complex, SecA undergoes repeated cycles of ATP-dependent conformational changes, which are linked to the movement of successive segments of a polypeptide chain through the channel<ref name=journal1/>. However the mechanism employed by SecA to translocate substrates polypeptide chains through the SecY channel remains largely unknown.

An important issue concerning the function of SecA is its oligomeric state during translocation. SecA is a dimer in solution<ref name=journal1/>, and previous work argued that this is its functional state<ref name=journal1/>. An x-ray structure of ''Bacillus subtilis'' SecA also indicates the existence of a dimer<ref name=journal1/>. However, recent evidence raises the possibility that SecA might actually function as a monomer; in solution, SecA dimers are in rapid equilibrium with monomers<ref name=journal1/>. Although the equilibrium favors dimers, it is shifted almost completely toward monomers in the presence of membranes containing acidic phospholipids or upon binding to the SecY complex<ref name=journal1/>. A synthetic signal peptide had a similar effect, although this result is controversial<ref name=journal1/>. A monomeric derivative of SecA containing six point mutations retained some in vitro translocation activity<ref name=journal1/>, but the low level of translocation precluded any firm conclusion. In addition, the previous results do not exclude models in which SecA cycles between monomeric and oligomeric states during the translocation of a polypeptide chain<ref name=journal1/>. Most importantly, the functional oligomeric state of SecA in vivo remains to be established.

In ''Bacillus subtilis'', the secretion of extracellular proteins strongly increases upon transition from exponential growth to the stationary growth phase. It is not known whether the amounts of some or all components of the protein translocation apparatus are concomitantly increased in relation to the increased export activity. In this study, we analyzed the transcriptional organization and temporal expression of the secA gene, encoding a central component of the ''B. subtilis'' preprotein translocase. We found that secA and the downstream gene (prfB) constitute an operon that is transcribed from a vegetative (A-dependent) promoter located upstream of secA. Furthermore, using different independent methods, we found that secA expression occurred mainly in the exponential growth phase, reaching a maximal value almost precisely at the transition from exponential growth to the stationary growth phase. Following to this maximum, the de novo transcription of secA sharply decreased to a low basal level. Since at the time of maximal secA transcription the secretion activity of ''B. subtilis'' strongly increases, our results clearly demonstrate that the expression of at least one of the central components of the ''B. subtilis'' protein export apparatus is adapted to the increased demand for protein secretion. Possible mechanistic consequences are discussed.<ref name=journal3>PMID:9882663</ref>  

=References=

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