crescentin
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Crescentin is a
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, res ...
which is a
bacterial Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were amon ...
relative of the
intermediate filaments Intermediate filaments (IFs) are cytoskeletal structural components found in the cells of vertebrates, and many invertebrates. Homologues of the IF protein have been noted in an invertebrate, the cephalochordate '' Branchiostoma''. Intermedi ...
found in
eukaryotic cells Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bact ...
. Just as tubulins and
actin Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of ov ...
s, the other major cytoskeletal proteins, have
prokaryotic A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Connec ...
homologs A couple of homologous chromosomes, or homologs, are a set of one maternal and one paternal chromosome that pair up with each other inside a cell during fertilization. Homologs have the same genes in the same loci where they provide points alon ...
in, respectively, the
FtsZ FtsZ is a protein encoded by the ''ftsZ'' gene that assembles into a ring at the future site of bacterial cell division (also called the Z ring). FtsZ is a prokaryotic homologue of the eukaryotic protein tubulin. The initials FtsZ mean "Filam ...
and
MreB MreB is a protein found in bacteria that has been identified as a homologue of actin, as indicated by similarities in tertiary structure and conservation of active site peptide sequence. The conservation of protein structure suggests the common a ...
proteins, intermediate filaments are linked to the crescentin protein. Some of its homologs are erroneously labelled Chromosome segregation protein ParA. This protein family is found in '' Caulobacter'' and ''
Methylobacterium ''Methylobacterium'' is a genus of Hyphomicrobiales.Garrity, George M.; Brenner, Don J.; Krieg, Noel R.; Staley, James T. (eds.) (2005). Bergey's Manual of Systematic Bacteriology, Volume Two: The Proteobacteria, Part C: The Alpha-, Beta-, Delta ...
''.


Role in cell shape

Crescentin was discovered in 2009 by Christine Jacobs-Wagner in ''
Caulobacter crescentus ''Caulobacter crescentus'' is a Gram-negative, oligotrophic bacterium widely distributed in fresh water lakes and streams. The taxon is more properly known as ''Caulobacter vibrioides'' (Henrici and Johnson 1935). ''C. crescentus'' is an importa ...
'' (now ''vibrioides''), an aquatic bacterium which uses its
crescent A crescent shape (, ) is a symbol or emblem used to represent the lunar phase in the first quarter (the "sickle moon"), or by extension a symbol representing the Moon itself. In Hinduism, Lord Shiva is often shown wearing a crescent moon on his ...
-shaped cells for enhanced motility. The crescentin protein is located on the
concave Concave or concavity may refer to: Science and technology * Concave lens * Concave mirror Mathematics * Concave function, the negative of a convex function * Concave polygon, a polygon which is not convex * Concave set * The concavity of a ...
face of these cells and appears to be necessary for their shape, as
mutants In biology, and especially in genetics, a mutant is an organism or a new genetic character arising or resulting from an instance of mutation, which is generally an alteration of the DNA sequence of the genome or chromosome of an organism. It ...
lacking the protein form rod-shaped cells. To influence the shape of the ''Caulobacter'' cells, the
helices A helix () is a shape like a corkscrew or spiral staircase. It is a type of smooth space curve with tangent lines at a constant angle to a fixed axis. Helices are important in biology, as the DNA molecule is formed as two intertwined helices, ...
of crescentin filaments associate with the cytoplasmic side of the
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 ( ...
on one lateral side of the cell. This induces a curved cell shape in younger cells, which are shorter than the helical pitch of crescentin, but induces a spiral shape in older, longer cells.


Protein structure

Like eukaryotic intermediate filaments, crescentin organizes into filaments and is present in a helical structure in the cell. Crescentin is necessary for both shapes of the ''Caulobacter'' prokaryote (vibroid/crescent-shape and helical shape, which it may adopt after a long stationary phase). The crescentin protein has 430 residues; its sequence mostly consists of a pattern of 7 repeated residues which form a coiled-coil structure. The DNA sequence of the protein has sections very similar to the eukaryotic
keratin Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up scales, hair, nails, feathers, ho ...
and
lamin Lamins, also known as nuclear lamins are fibrous proteins in type V intermediate filaments, providing structural function and transcriptional regulation in the cell nucleus. Nuclear lamins interact with inner nuclear membrane proteins to form t ...
proteins, mostly involving the coiled-coil structure. Researchers Ausmees et al. recently proved that, like animal intermediate filament proteins, crescentin has a central rod made up of four coiled-coil segments. Both intermediate filament and crescentin proteins have a primary sequence including four α-helical segments along with non-α-helical linker domains. An important difference between crescentin and animal intermediate filament proteins is that crescentin lacks certain consensus sequence elements at the ends of the rod domain which are conserved in animal lamin and keratin proteins. The protein has been divided into a few subdomains organized similarly to eukaryotic IF proteins. Not every researcher is convinced that it is a homolog of intermediate filaments, suggesting instead that the similarity might have arisen via convergent evolution.


Assembly of filaments

Eukaryotic intermediate filament proteins assemble into filaments of 8-15 nm within the cell without the need for energy input, that is, no need for ATP or GTP. Ausmees et al. continued their crescentin research by testing whether the protein could assemble into filaments in this manner ''
in vitro ''In vitro'' (meaning in glass, or ''in the glass'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called " test-tube experiments", these studies in biology ...
''. They found that crescentin proteins were indeed able to form filaments about 10 nm wide, and that some of these filaments organized laterally into bundles, just as eukaryotic intermediate filaments do. The similarity of crescentin protein to intermediate filament proteins suggests an
evolutionary Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
linkage between these two cytoskeletal proteins. Like eukaryotic intermediate filaments, the filament built from crescentin is elastic. Individual proteins dissociate slowly, making the structure somewhat stiff and slow to remodel. Strain does not induce hardening of the structure, unlike eukaryotic IFs that do.


References

{{Cytoskeletal Proteins Cytoskeleton Bacterial proteins