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From CellBiology

Individual Project - Spectrin

Spectrin in Erythrocyte Membrane Cytoskeleton

Various actin-binding proteins (ABP) are known to be found in the nucleus of cells and are responsible for the regulation of actin filaments in the cytoplasm. Cellular processes such as chromatin remodeling, RNA processing, transcription and nuclear export greatly rely on nuclear actin. Actin-binding proteins thus play a significant role in the regulation of processes in the nucleus and the entire cell. An example of an actin binding protein is spectrin.[1]


Spectrin is a member of the cytoskeleton protein family; it is most commonly associated with the plasma membrane. Cytoskeletal proteins are classified into two types: cross-linking actin filaments and ones which link actin filaments to the cell membrane. Spectrin belongs to the latter group. It can be found in most vertebrate tissue and certain invertebrates. Spectrin isoforms can bind to a variety of ligands, including metal ions, fatty acids, phospholipids and proteins. It also provides the fundamental structural meshwork for erythrocyte membranes. Therefore researchers have used erythrocytes as a standardized model to explore the properties of this protein. [2][3][4]


Table of actin cross-linking proteins

Location

• Two types of alpha subunits are present mammals, these are products of distinct genes

• Located on chromosome 9 in all mammalian tissues except mature erythrocytes

• Located on chromosome 1 in mature erythrocytes

• Birds and other animals are presumed to have single alpha subunits expressed in erythrocytes

[5][6]

Structure

Simplified Diagram of Spectrin


The spectrin protein is a rod shaped, flexible molecule ranging from 200 to 260 nm in length. It is comprised of two non-identical subunits, α and β subunits, with molecular weights of 280 and 246 Kd respectively. Its side by side alignment forms the protein’s heterodimers, and a head to head alignment gives its tetramer structures. In many cases, there is a calmodulin binding site near the center of the alpha subunit of spectrins. (Note: this feature is absent in mammalian erythrocytes) Likewise, ankyrin binding sites are found in the middle of the beta subunits of the tetramers. Under high resolution electron microscopy, we will be able to see five to six rod shaped spectrin molecules linked to short actin filaments ranging from 30 to 50 nm in length. The linkage of these two develops into a sheet consisting of polygons and hexagons. [7][8][9]


The primary structure of spectrin contains subunits of a 106-amino acid repeating motif. It is believed that each recurrent sequence invaginates into three α helixes, namely the spectrin repeat.


Link to Spectrin structure movie

Function

Spectrins are responsible for the connection of actin filaments and important membrane proteins. It is a tetramer that interlinks transmembrane proteins, membrane lipids and the actin cytoskeleton. This connection can be direct or through adaptor proteins such as ankyrin and 4.1. It’s specialization in dimer synthesis contributes to the functional molecular architecture as well. As a cytoskeleton protein, it also contributes to cell support, cell membrane stabilization, locomotion as well as maintaining and restoring the overall cell shape. They are also assumed to be responsible for providing routes for intercellular traffic, dynamic exchange of subunits, connecting different systems by multi-specific cross-linking proteins and rapid remodeling in some cases. Nonerythroid spectrins play a role in cell adhesion, cell polarity generation and attaching other cytoskeletal structures to the plasma membrane. [10][11][12][13]


It is believed that spectrin is also utilized in the formation of long, extended molecules, due to the fact that it usually occurs in multiple copies. They control specific distance between functional domain at the N- and C- termini. Spectrins also perform the role of signal transduction protein. In general, spectrin plays various important roles in cell regulations and many assumptions of its function have been based on findings of spectrins function in erythrocyte membrane skeleton.[14][15][16][17]


In summary, the major functions of spectrin are:

• Maintaining and restoring cell shape

• Linking actin filaments to cell membrance

• Alternative pathways for intercellular traffic

• Dynamic exchange of subunits

• Linking of systems by other multi-specific cross-linking proteins

• Signal transduction

Current Research

Previous research on spectrin has mostly been focused on the discovery of spectrin being the main constituent of erythrocyte membrane skeletons, as well as investigations of the major constituent proteins along with spectrin. Current research is aimed at exploring the role of spectrin in more complex cells and the cause and effects of spectrin mutants. In the future, it would be a challenge to explain the phenotypes of the molecular activities of spectrin.[18]

References

1. Bennett V and Lambert S (1991). The Spectrin Skeleton: From Red Cells to Brain. The Amercian Society for Clinical Investigation. 87:1483-1489

2. Djinovic-Carugo K, Gautel M, Ylanne J and Young P (2002). The spectrin repeat: a structural platform for cytoskeletal protein assemblies. Federation of European Biochemical Societies. Elsevier Science. 513: 119-123

3. Dubreuil R. R and Grushko T (1998). Genetic studies of spectrin: new life for a ghost protein. BioEssays. John Wiley & Sons. 20: 875-878

4. Forget B.G, Winkelmann J.C (1993). Erythroid and nonerythroid spectrins. Blood. American Society of Hematology. 81:3171-3185

5. Fukalova J, Filimonenko V and Hozak P. Actin-binding protein in the nucleus. Institute of Molecular Genetics, Dept. of Biology of the Cell Nucleus, Academy of Sciences of the Czech Republic.

6. Gratzer W B (1981). The red cell membrane and its cytoskeleton. Biochemistry Journal 198: 1-8

7. Matteis M A D and Morrow J S (1998). The role of ankyrin and spectrin in membrane transport and domain formation. Current Opinion in Cell Biology. Current Biology. 10: 542-549

8. Ungewickell E and Gratzer W (1978). Self-Association of Human Spectrin, A Thermodynamic and Kinetic Study. Eur. J. Biochem. 88: 379-385

9. Viel A and Branton D (1996). Spectrin: on the path from structure to function. Current Opinion in Cell Biology. 8: 49-55

Homework

Lecture 4 - Nucleus

What did you find interesting and did not know about the nucleus?


From the nucleus lecture, I have learnt that the nucleus consists of two concentric membranes, which form the nuclear envelope. It was interesting to know that they break down during mitosis and also contain nuclear pores that make them permeable.


Lecture 5 - Exocytosis

What concept about exocytosis did you find difficult to understand?

I found it hard understanding the transport between secretory compartments

Lecture 7 - Mitochondria

What types of cellular processes require lots of energy from the mitochondria?


The mitochondria produce energy (ATP) using energy that is stored in food by aerobic respiration. After glycolysis, pyruvate enters the mitochondrion to enter the Krebs cycle in order to complete the oxidation of organic fuel and finally produce ATP. Cellular processes such as signaling of cells, transportation of cells across cell membranes, cell differentiation, biosynthesis and locomotion require large amounts of the energy produced from the mitochondria.


Lecture 8 - Adhesion

What do the different "CAM" acronyms stand for?


CAM - cell adhesion molecules

Ng-CAM stands for Neuroglia Cell Adhesion Molecule

I-CAM stands for Intercellular Cellular Adhesion Molecule

L-CAM stands for Liver Cell Adhesion Molecule


Lecture 10

What is the name of the epidermal layer between the basal and granulosa layer and how does it relate to intermediate filaments?


The name of the epidermal layer between the basal and granulosa layer is called the stratum spinosum. Intermediate filaments are synthesized by cells of the stratum spinosum. These filaments help support the structure and also resists abrasion of the skin.


Lab 6 - Cytoskeleton Exercise

"If you've seen differences in the distribution of phenotypes in Tm4 over-expressing B35 cells versus control B35 cells, describe these differences. Formulate a hypothesis with regards to what changes on the molecular level may have occurred due to the over-expression of Tm4 that lead to morphological changes that you have observed"

From the phenotype lab, it could be observed that genotype A had more neuritis and lamella than genotype B. The cells of genotype B were also comparatively smaller than those of genotype A. Genotype A: Tm4 over-expressing B35 cells Genotype B: Wild type B35 cells

Lecture 14 - Confocal Microscopy

What are the 2 main forms of generating confocal microscopy?

The two main forms of generating confocal microscopy are:

1. laser scanning confocal microscopy

2. spinning disk confocal microscopy

Lecture 15 - Cell Cycle

What does "S" stand for in the S phase?


The S phase stands for the synthesis phase. It occurs during interphase in between G1 phase and the G2 phase.
  1. Fukalova J, Filimonenko V and Hozak P. Actin-binding protein in the nucleus. Institute of Molecular Genetics, Dept. of Biology of the Cell Nucleus, Academy of Sciences of the Czech Republic.
  2. Ungewickell E and Gratzer W (1978). Self-Association of Human Spectrin, A Thermodynamic and Kinetic Study. Eur. J. Biochem. 88: 379-385
  3. Gratzer W B (1981). The red cell membrane and its cytoskeleton. Biochemistry Journal 198: 1-8
  4. Forget B.G, Winkelmann J.C (1993). Erythroid and nonerythroid spectrins. Blood. American Society of Hematology. 81:3171-3185
  5. Matteis M A D and Morrow J S (1998). The role of ankyrin and spectrin in membrane transport and domain formation. Current Opinion in Cell Biology. Current Biology. 10: 542-549
  6. Viel A and Branton D (1996). Spectrin: on the path from structure to function. Current Opinion in Cell Biology. 8: 49-55
  7. Djinovic-Carugo K, Gautel M, Ylanne J and Young P (2002). The spectrin repeat: a structural platform for cytoskeletal protein assemblies. FEBS Letter. 513: 119-123
  8. Dubreuil R. R and Grushko T (1998). Genetic studies of spectrin: new life for a ghost protein. BioEssays. John Wiley & Sons. 20: 875-878
  9. Matteis M A D and Morrow J S (1998). The role of ankyrin and spectrin in membrane transport and domain formation. Current Opinion in Cell Biology. Current Biology. 10: 542-549
  10. Ungewickell E and Gratzer W (1978). Self-Association of Human Spectrin, A Thermodynamic and Kinetic Study. Eur. J. Biochem. 88: 379-385
  11. Djinovic-Carugo K, Gautel M, Ylanne J and Young P (2002). The spectrin repeat: a structural platform for cytoskeletal protein assemblies. FEBS Letter. 513: 119-123
  12. Dubreuil R. R and Grushko T (1998). Genetic studies of spectrin: new life for a ghost protein. BioEssays. John Wiley & Sons. 20: 875-878
  13. Matteis M A D and Morrow J S (1998). The role of ankyrin and spectrin in membrane transport and domain formation. Current Opinion in Cell Biology. Current Biology. 10: 542-549
  14. Ungewickell E and Gratzer W (1978). Self-Association of Human Spectrin, A Thermodynamic and Kinetic Study. Eur. J. Biochem. 88: 379-385
  15. Djinovic-Carugo K, Gautel M, Ylanne J and Young P (2002). The spectrin repeat: a structural platform for cytoskeletal protein assemblies. FEBS Letter. 513: 119-123
  16. Dubreuil R. R and Grushko T (1998). Genetic studies of spectrin: new life for a ghost protein. BioEssays. John Wiley & Sons. 20: 875-878
  17. Matteis M A D and Morrow J S (1998). The role of ankyrin and spectrin in membrane transport and domain formation. Current Opinion in Cell Biology. Current Biology. 10: 542-549
  18. Ungewickell E and Gratzer W (1978). Self-Association of Human Spectrin, A Thermodynamic and Kinetic Study. Eur. J. Biochem. 88: 379-385