Pilin
Encyclopedia
Pilin refers to a class of fibrous protein
s that are found in pilus
structures in bacteria
. Bacterial pili are used in the exchange of gene
tic material during bacterial conjugation
, and a short pilus called a fimbrium is used as a cell adhesion
mechanism. Although not all bacteria have pili or fimbriae, bacterial pathogen
s often use their fimbriae to attach to host cells. In gram-negative
bacteria, where pili are more common, individual pilin molecules are linked by noncovalent protein-protein interaction
s, while gram-positive
bacteria often have polymer
ized pilin.
Pilin proteins themselves are α+β proteins characterized by a very long N-terminal alpha helix
. Many pilins are post-translationally modified by glycosylation
or phosphorylation
. The assembly of a complete pilus relies on interactions between the N-terminal helices of the individual monomers. The pilus structure sequesters the helices in the center of the fiber lining a central pore, while antiparallel beta sheet
s occupy the exterior of the fiber. The exact mechanism of pilus assembly from monomers is not known, although chaperone proteins have been identified for some types of pilin. and specific amino acid
s required for proper pilus formation have been isolated.
. These bonds provide enhanced stability to the protein. Recently, the pilin protein from Streptococcus pyogenes
has been split in to two fragments to develop a new molecular tool called the isopeptag
. The isopeptag
is a short peptide that can be attached to a protein of interest and can bind its binding partner through a spontaneously formed isopeptide bond
. This new peptide tag can allow scientists to target and isolate their proteins of interest through a permanent covalent bond.
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s that are found in pilus
Pilus
right|thumb|350px|Schematic drawing of bacterial conjugation. 1- Donor cell produces pilus. 2- Pilus attaches to recipient cell, brings the two cells together. 3- The mobile plasmid is nicked and a single strand of DNA is then transferred to the recipient cell...
structures in bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
. Bacterial pili are used in the exchange of gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
tic material during bacterial conjugation
Bacterial conjugation
Bacterial conjugation is the transfer of genetic material between bacterial cells by direct cell-to-cell contact or by a bridge-like connection between two cells...
, and a short pilus called a fimbrium is used as a cell adhesion
Cell adhesion
Cellular adhesion is the binding of a cell to a surface, extracellular matrix or another cell using cell adhesion molecules such as selectins, integrins, and cadherins. Correct cellular adhesion is essential in maintaining multicellular structure...
mechanism. Although not all bacteria have pili or fimbriae, bacterial pathogen
Pathogen
A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...
s often use their fimbriae to attach to host cells. In gram-negative
Gram-negative
Gram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color...
bacteria, where pili are more common, individual pilin molecules are linked by noncovalent protein-protein interaction
Protein-protein interaction
Protein–protein interactions occur when two or more proteins bind together, often to carry out their biological function. Many of the most important molecular processes in the cell such as DNA replication are carried out by large molecular machines that are built from a large number of protein...
s, while gram-positive
Gram-positive
Gram-positive bacteria are those that are stained dark blue or violet by Gram staining. This is in contrast to Gram-negative bacteria, which cannot retain the crystal violet stain, instead taking up the counterstain and appearing red or pink...
bacteria often have polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
ized pilin.
Pilin proteins themselves are α+β proteins characterized by a very long N-terminal alpha helix
Alpha helix
A common motif in the secondary structure of proteins, the alpha helix is a right-handed coiled or spiral conformation, in which every backbone N-H group donates a hydrogen bond to the backbone C=O group of the amino acid four residues earlier...
. Many pilins are post-translationally modified by glycosylation
Glycosylation
Glycosylation is the reaction in which a carbohydrate, i.e. a glycosyl donor, is attached to a hydroxyl or other functional group of another molecule . In biology glycosylation refers to the enzymatic process that attaches glycans to proteins, lipids, or other organic molecules...
or phosphorylation
Phosphorylation
Phosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....
. The assembly of a complete pilus relies on interactions between the N-terminal helices of the individual monomers. The pilus structure sequesters the helices in the center of the fiber lining a central pore, while antiparallel beta sheet
Beta sheet
The β sheet is the second form of regular secondary structure in proteins, only somewhat less common than the alpha helix. Beta sheets consist of beta strands connected laterally by at least two or three backbone hydrogen bonds, forming a generally twisted, pleated sheet...
s occupy the exterior of the fiber. The exact mechanism of pilus assembly from monomers is not known, although chaperone proteins have been identified for some types of pilin. and specific amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
s required for proper pilus formation have been isolated.
Development of Molecular Tools
Pili in Gram-positive bacteria contain spontaneously formed isopeptide bondsIsopeptide bond
An isopeptide bond is an amide bond that is not present on the main chain of a protein. The bond forms between the carboxyl terminus of one protein and the amino group of a lysine residue on another protein....
. These bonds provide enhanced stability to the protein. Recently, the pilin protein from Streptococcus pyogenes
Streptococcus pyogenes
Streptococcus pyogenes is a spherical, Gram-positive bacterium that is the cause of group A streptococcal infections. S. pyogenes displays streptococcal group A antigen on its cell wall. S...
has been split in to two fragments to develop a new molecular tool called the isopeptag
Isopeptag
Isopeptag is a 16 amino acid peptide tag that can be genetically linked to proteins without interfering with protein folding. What makes the isopeptag different from other peptide tags is that it can bind its binding protein through a permanent and irreversible covalent bond...
. The isopeptag
Isopeptag
Isopeptag is a 16 amino acid peptide tag that can be genetically linked to proteins without interfering with protein folding. What makes the isopeptag different from other peptide tags is that it can bind its binding protein through a permanent and irreversible covalent bond...
is a short peptide that can be attached to a protein of interest and can bind its binding partner through a spontaneously formed isopeptide bond
Isopeptide bond
An isopeptide bond is an amide bond that is not present on the main chain of a protein. The bond forms between the carboxyl terminus of one protein and the amino group of a lysine residue on another protein....
. This new peptide tag can allow scientists to target and isolate their proteins of interest through a permanent covalent bond.