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TUBULIN, ELASTIN, ENAMEL

TUBULIN

       Tubulins are required for the spindles which form in mitosis and meiosis, for the transport along axons, for cilia and flagella, and for the correct localization of organelles.  (Cilia are evident in the epidermis of the worm and in the lining of the mammalian trachea depicted in the following images).

WORM
PSEUDOSTRAT

Microtubules are hollow structures which are usually composed of 13 filaments arranged in parallel.  All eukaryotes appear to require α, β, and γ tubulins and most multicellular eukaryotes possess multiple α and β tubulin genes (Dutcher, 2001). Although tubulin and other cytoskeletal proteins (such as actin) were once thought to be specific to eukaryotes, their homologs have been identified in bacteria which lack a true eukaryotic cytoskeleton.Bacterial homologs of tubulin and actin are structurally and functionally similar to those of eukaryotes (Møller-Jensen, 2005).

In bacterial cell division, a structure known as the Z ring constricts the cell.   This ring is composed of FtsZ, which is homologous to tubulin which constricts eukaryotic cells during division (Amos, 2004). FtsZ is known in eubacteria, archaea and some mitochondria (Gull, 2001; Brown, 1997b). Polymerization of FtsZ is one of the earliest events in the division of bacteria. In bacteria, FtsZ polymers form both long fibers ans sircular structures (Lafontaine, 2007). A similar structure is formed from the spindle early in cytokinesis in plants and yeast.  In all eukaryotes, the mitotic spindle’s equator identifies the site for cytokinesis (van den Ent, 2001; Leung, 2004). FtsZ and MreB form helical structures inside the bacterial cell membrane (Møller-Jensen, 2005).Chloroplasts and some primitive mitochondria produce their own FtsZ, MinD, and MinE for their division while most mitochondria depend on mechanisms of the host nucleus (Amos, 2004; Oysteryoung, 2001).  Although the three-dimensional structures of tubulin and FtsZ are highly similar, the amino acid sequences share limited homology (Pogliano, 2008). Other tubulin homologs are also known in bacteria and archaea. BtubA and BtubB form heterodimers, just as their eukaryotic homologs α and β tubulin. Other homologs of tubulin include TubZ and RepX (Pogliano, 2008).

     Tubulin is the primary component of the cytoskeleton in Giardia, which possess two α tubulin genes and three β tubulin genes (Elmendorf, 2003).  In many, but not all, eukaryotes α and β tubulins can be modified after translation through the addition of additional chains of glycyl and glutamyl residues (polyglcylation and polyglutamylation) (Gull, 2001).  Both polyacylation and polyglutamylation are modifications of tubulin which occur in Giardia.

Giardia seem to lack γ tubulin presence in the microtubule organizing centers, although the gene is expressed in the cell (Elmendorf, 2003).

 

One of the ways in which TNF causes cell death may be in the modification of tubulin by tubulin tyrosine ligase (TTL) forming a nonfunctional cytoskeleton (Idriss, 2004).

α tubulin forms a heterodimer with β tubulin to form microtubules.

 

β tubulin

 

γ tubulin is required for the polymerization of α and β subunits to form a heterodimer in the formation of microtubules.  It is located in microtubule organizing centers (MTOCs).  Null mutations in this gene are lethal.  Without γ tubulin, a flagellum can form but it is unable to function (McKean, 2003).

 

There are additional tubulins which are known in many, but not all eukaryotes.  Humans have δ tubulin and ε tubulin genes (Dutcher, 2001).

 

δ tubulin is required for normal flagella in algae (such as in Euglena  in the following image) and normal basal bodies in protists.

ALGAE

η tubulin mutations cause abnormal basal bodies in protists.

 

ζ tubulin is associates with basal bodies of trypanosomes and the centrioles of some animal cells.

 

ε tubulin is located in centrosomes.

The γ-, ε-, η-, and δ-tubulins are needed for centriole duplication and assembly (Beisson, 2003).

 

     The flagella of archaea lack any proteins which compose eubacterial flagella and are unique structures.  Archaeal flagellar proteins are similar to pilins (Thomas, 2001).

 

 

     Although cytokinesis is typically depicted as occurring in a “purse-string” fashion in all eukaryotic cells, there are a number of variations on the mechanisms conventional cytokinesis, particularly in protists and plants (Uveda, 2004; Otegui, 2000).

One of the enzymes involved in modifying tubulin during microtubule formation is the enzyme tubulin tyrosine ligase (Y-ligase). It shares a protein fold with enzymes of the glutathione synthetase ADP-forming family (Dideberg, 1998).

 

THE PROTEINS OF ENAMEL

     The proteins of vertebrate enamel are not expressed in any other parts of the body and their function is unique, even among other biomineralized tissues.  Amelogenin is the major protein in vertebrate enamel, composing 90% of the organic portion of enamel.  One of its exons (exon 2) is homologous to an exon of proteins found in protostomes (osteonectin) and deuterosotmes (SC1, hevin, and QR1).  Enamel definitely existed in Ordovician ostracoderms while its existence in Cambrian euconodonts and fish such as Anatolepis is less certain (Delgado, 2001).

     The secretory calcium-binding phosphoprotein family (SCPP) includes three proteins in enamel matrix (amelogenin, enamelin, and ameloblastin), five proteins involved in the formation of dentin and bone (dentin, sialophosphoprotein, dentin matrix acidic phosphoprotein 1, integrin-binding sialoprotein, matrix extracelullar phophoglycoprotein, and secreted phosphoprotein 1), caseins, and several salivary proteins.  Most of these genes are located on a cluster on chromosome 4q13 in humans.  This gene family seems to have arisen from the SPARC gene.  SPARC, which is expressed in fish bone and scales, may have been the first gene expressed in vertebrate mineralized tissue.  SPARC is expressed where the epithelium meets the connective tissue beneath in invertebrates and jawless fish (Kawasaki, 2004, 2003). 

    The most primitive forms of cartilage in lancelets and jawless fish does not involve collagen.   SPARC is only associated with collagenous skeletal tissues.  SPARCL1 gave rise to amelogenin, enamelin, and ameloblastin early in the history of gnathostomes (Kawasaki, 2004).

 

Casein is a milk protein whose original function may have been a protective egg coat.  As it began to be used as a food source in mammals, other genes such as α-lactalbumin (which resulted from a duplication of lysozyme) were also expressed in milk (Kawasaki, 2003).

 

ELASTIN AND CARTILAGE

Originally, jawless fishes were thought to possess true cartilage and certain invertebrate tissues were either classified as condroid if they were similar to cartilage or chordoid if they were similar to the tissue of the notochord.  Given that lamprey and hagfish cartilages are now known not to possess collagen as a major component, any definition which includes the skeletal tissues of jawless fish as true cartilage also includes a number of invertebrate cartilages.  Cartilage/cartilage-like tissue is known in cnidarians, annelids, arthropods, and mollusks (Robson, 1999). 

  The cartilage of squid does possess collagen and resembles the hyaline cartilage of vertebrates, although it is not the collagen II found in vertebrate hyaline cartilage.  A variety of collagenous and non-collagenous proteins are known from invertebrate cartilages (excluding type II collagen) which seem to have evolved separately in different lineages.  Cartilaginous fish are the most primitive animals which produce cartilage with collagen type II (and type I). Hyaline cartilage in most vertebrates possesses collagen II as its major protein with lesser amounts of collagen types VI, IX, XI, and XII.

Lampreys possess cartilaginous arches above the notochord.

Although type II collagen exists in the notochord of lampreys and hagfish, it is not a major component of agnathan cartilage.  The major protein in lamprey cartilage is named lamprin, that of hagfish cartilage is named myxinin.   Lamprin is homologous to vertebrate elastin, insect chorion proteins, and spider silk proteins (Robson, 1999). 

     Since only about 5% of the amino acids in elastin are charged, it is one of the most hydrophobic proteins known (Robson, 1999).  All gnathostomes possess elastic fibers in the aortic wall. Higher gnathostomes modified elastin to make it increasingly hydrophobic (Chalmers, G. W., 1999).

 

Inveretbrates possess a large complex dystrophin/urotrophin gene which is homologous to the dystrophin and urotrophin genes found in vertebrates (Neuman, 2005).