The SecA protein is a

cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...

associated subunit of the

eubacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...

l Sec or Type II

secretory pathway 440px Secretion is the movement of material from one point to another, such as a secreted chemical substance from a cell or gland. In contrast, excretion is the removal of certain substances or waste products from a cell or organism. The classica ...

, a system which is responsible for the secretion of proteins through the cell membrane. Within this system the SecA

ATPase ATPases (, Adenosine 5'-TriPhosphatase, adenylpyrophosphatase, ATP monophosphatase, triphosphatase, SV40 T-antigen, ATP hydrolase, complex V (mitochondrial electron transport), (Ca2+ + Mg2+)-ATPase, HCO3−-ATPase, adenosine triphosphatase) are ...

forms a

translocase Translocase is a general term for a protein that assists in moving another molecule, usually across a cell membrane. These enzymes catalyze the movement of ions or molecules across membranes or their separation within membranes. The reaction is des ...

complex with the SecYEG channel, thereby driving the movement of the

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

substrate across the membrane.

Structure

SecA is a complex protein whose structure consists of six characterized domains that can explain SecA's capabilities to bind substrates and to move them. The following five domains seem to be present in all SecA proteins that have been structurally analyzed so far.

DEAD motor domain

This

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...

domain is subdivided into the two

nucleotide Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecules wi ...

binding folds 1 and 2 (NBF1 and NBF2) where ATP is bound and hydrolyzed. The chemical energy from the phosphodiester bonds results in a conformational change which is transferred to other domains (especially the HWD and the PPXD domains) which consequently mechanically move the preprotein across the membrane. However, these conformational changes are partly regulated by other

protomer In structural biology, a protomer is the structural unit of an oligomeric protein. It is the smallest unit composed of at least two different protein chains that form a larger hetero-oligomer by association of two or more copies of this unit. The ...

domains described below.

C-terminal linker domain

The capability to bind to the SecB chaperone during

post-translational Post-translational modification (PTM) is the covalent and generally enzymatic modification of proteins following protein biosynthesis. This process occurs in the endoplasmic reticulum and the golgi apparatus. Proteins are synthesized by ribosom ...

translocation, the ribosome (during both post-translational translocation and co-translational translocation ) and the

phospholipid Phospholipids, are a class of lipids whose molecule has a hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an alcohol residue (usually a glycerol molecule). Marine phospholipids typ ...

bilayer is important for SecA functioning and is achieved by the C-terminal linker domain.

Helical wing domain (HWD)

Located at the C-terminal portion of the molecule, this domain is in contact with the HSD and PPXD domains. Likely it plays a role in transferring molecular conformational motion, which it receives from HSD and which originates from ATP hydrolysis in the

DEAD motor domain Death is the irreversible cessation of all biological functions that sustain an organism. For organisms with a brain, death can also be defined as the irreversible cessation of functioning of the whole brain, including brainstem, and brain ...

, to the PPXD domain.

Peptide cross linking domain (PPXD)

Since SecA's essential function is the transport of preprotein across the membrane the ability to actually bind preprotein must be given. The PPXD domain fulfils this function upon substrate binding.

Helical scaffold domain (HSD)

This domain lies in the center of the SecA

and contacts via α-helical interactions all other subdomains. In addition it contains the intramolecular regulator of ATP hydrolysis 1 (IRA1) subdomain which seems to prevent unwanted ATP hydrolysis when SecA is not bound to SecYEG. Together with IRA1, a conserved salt bridge called Gate 1 might function to prevent unnecessary conformational change. Gate 1 seems to functionally connect the nucleotide ( ATP) binding site of the

with the PPXD domain which results in regulation of ATP hydrolysis only upon preprotein binding. However, this coordinative behaviour has only been shown to occur when SecA is bound to SecYEG.

References

{{reflist Protein targeting Protein domains Protein families Transmembrane proteins Secretion