Introduction
As a multi-functional protein, Cytoplasmic Actin can be found in all eukaryotic cells at higher concentration levels of over 100 μM. It is the most abundant protein found in most eukaryotic cells, which makes it a critical player in different cellular functions, such as maintaining cell shape and polarity to transcription regulation. The structure of Cytoplasmic Actin is distinguished by a large number of polymorphic proteins which form the filaments. In eukaryotic organisms, such as archaea and eubacteria, actin-like proteins activities are related to sub-cellular organization, motility, cell shape maintenance and cell cycle progression; they are either associated to cell membrane or freely soluble. In this paper, we will discuss the primary, secondary and tertiary structures of the protein, the structures of the monomer and filament, and how this protein assembles and disassembles; we will indicate where the protein is found in the cell, and how its location and structure are related to its different functions.
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Different Structures of the Protein
Actins can be found throughout the cytoplasm of eukaryotic organisms, it comprises 20 percent of total cellular protein. There are four different structures of protein, primary, secondary, tertiary and quaternary structure; in this paper we will discuss the first three structures in details.
Primary Structure
It comprises all covalent bonds of the molecule which are formed during protein biosynthesis process; it takes the shape of a group of amino acids of the peptide or protein, which is determined by the sequence of nucleotides in the gene encoding it. The backbone of this structure contains deoxyriboses; they are linked together by phosphodiester bridges. The primary structure is distinguished by ten rational angles which render the backbone flexible. During a process called translation, a sequence of nucleotides in DNA is transcribed into mRNA; it is a unique sequence to that protein, it has the role of defining the structure and function of the protein. We can read this sequence directly from the sequence of the gene using the genetic code, or, we can use other methods like tandem mass spectromy or Edman degradation. All subsequent levels of protein structure are dependent on primary structure.
Secondary Structure
Alpha helices and beta sheets are the common forms of the polypeptide in the secondary structure, they are generated through regular hydrogen-bonding interactions between N–H and C=O groups in the invariant parts of the amino acids in the polypeptide backbone. It is the local structure assumed by a portion of polypeptide via regular hydrogen bonds of the biopolymer, as observed in an atomic-resolution structure. Basically, this structure reflects the steric relationship (inter-strand and intra-strand steric relationships) of bases close to each other in the linear arrangement of nucleotides.
Tertiary Structure
The tertiary structure of protein represents the steric relationship of segments of DNA that are apart in the linear sequence. As the alpha-helices and beta-sheets are folded into a compact globule, we call this structure the overall folding of the polypeptide. It refers to three-dimensional structure of a single protein molecule. It embodies different kinds of global interactions that determine tertiary structure occur between the R-groups of the amino acids, such as disulfide bonds, ionic bonds, hydrogen bonds and hydrophobic interactions; the balance between them determines and stabilizes the conformation of a polypeptide.
Monomers and Filaments
A monomer is a small molecule, mostly organic, that uses chemical activity to be bonded to other molecules so they can together form a polymer, such as glucose, which is linked by glycoside bonds into polymers. A single polymer molecule may consist of hundreds to a million monomers. Actually, monomers have two distinctive features: carbon-carbon double bonds and side groups. Some types of monomers, such as DsRed-Monomer, contain forty-five amino acid substitutions.
A filament is a thin, flexible fiber with a 7 nm diameter and length that may extend to several micrometers. Actin filament has a distinct polarity; we can clearly distinguish their plus and minus ends from one another. This polarity of actin filaments is important in their assembly. They are capable of creating a unique direction of myosin movement relative to actin. Actin filaments depolymerize to monomers in solutions of low ionic strength, so that actin filaments can grow by the reversible addition of monomers to both ends.
How this protein assembles and disassembles
Actin is a globular protein that polymerize into extended filaments. The assembly and disassembly of actin filaments occurs during a dynamic process called cellular motility. The disassembly of the cytoplasmic microtubules and the assembly of the spindle apparatus is part of the mitotic cycle, such as cell division. Moreover, intermediate filament proteins are capable of being assembled into dimers via coiled-coil interactions.
The protein gelsolin is considered a key regulator in the assembly and disassembly of actin. The monomers of actin filaments are not assembled by strong bonds. This situation can be solved by the available lateral bonds with neighboring monomers, which are capable of being broken by thermal agitation. However, this weak bond can be helpful in allowing filaments to easily release or incorporate monomers, so that filaments can be reshaped in short time, which in its turn help changing the cellular structure according to different environmental stimulus.
Protein Location, Structure and Functionality
As a key component of the cytoskeleton in all eukaryotic creatures, it can be found in all eukaryotic organisms, most of these organisms contain many cytoplasmic actin genes. It determines how the cell can move, how its shape looks like while still playing critical roles in many other processes, such as organelle transport. It is the most abundant protein inside cells, it is made up of a collection of large protein cables or filaments that give cells shape and structure, and confer upon cells the ability to change shape and move. It facilitates every chemical reaction that occurs in a cell and the transport of many small molecules in and out of the cell.
Actins are polymers, large-size molecules consisting repeating units that are capable of binding, chemically reacting and switching. They are flexible protein molecules that can change in response to different stimulus. In their structure, they are made up of polypeptide chains, which are known by amino acids, bonded together with peptide bonds. A group of cells can show actin networks. During inter-phase, plant cells contain extensive networks of actin-containing bundles; they are present in the cytoplasm.
Nuclear Functions of Actin
Early studies imply that actins can also exist in the cell nucleus and is implicated in the expression of protein-coding genes. Actins have an essential role in different processes in the cell nucleus. Moreover, as a component of chromatin-modifying complexes, actin participates in gene expression. As a part of chromatin remodeling complexes, actin is associated with the transcription machineries, it influences long-range chromatin organization. As a major component of the cytoskeleton, actins determine cell migration and intracellular trafficking.
Conclusion
This paper focuses on Cytoplasmic Actins, as a protein that can be found in all eukaryotic organisms. We discussed the different structures of actins, primary, secondary and tertiary structures; we also discussed monomers and filaments as essential components of actins. Finally we discussed the position of actins and where we can find them, their different functions and roles in various biological processes. References
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