What is the Difference Between Histone and Nonhistone Protein

The main difference between histone and nonhistone protein is that histone protein packages the DNA into structural units known as nucleosomes whereas nonhistone protein includes the proteins remain in chromatin after the histones have been removed. Furthermore, histones are the chief protein component of chromatin while nonhistone proteins include scaffold proteins, heterochromatin protein 1, Polycomb, and DNA polymerase. 

Histone and nonhistone protein are two types of proteins present in the chromatin structure of DNA. They perform various functions related to DNA. 

Key Areas Covered 

1. What is Histone Protein
     – Definition, Types, Function
2. What is Nonhistone Protein
     – Definition, Types, Function
3. What are the Similarities Between Histone and Nonhistone Protein
     – Outline of Common Features
4. What is the Difference Between Histone and Nonhistone Protein
     – Comparison of Key Differences

Key Terms 

Chromatin, DNA, Histone Protein, Nonhistone Protein, Nucleosomes 

What is Histone Protein 

Histone protein is a family of highly alkaline proteins present in the nucleus of eukaryotic cells. Their positive charge facilitates the association with negatively charged DNA. The main function of histone proteins is to package and order the DNA into structural units called nucleosomes. Chromatin refers to the pack of nucleosomes. Therefore, they serve as spools around which DNA winds. Hence, histones are a major component of chromatin. However, unwound DNA without histones would be very long; 1.8 meters in a human cell. But, even the duplicated DNA condensed with chromatin results in 120 micrometers long chromosomes with histone packaging. Also, they play a key role in gene regulation by involving histone modifications.  

Figure 1: Formation of a Nucleosome

Furthermore, the five classes of histones are H1 (or H5), H2A, H2B, H3, and H4. Here, the H2A, H2B, H3, and H4 serve as core histones while H1 and H5 serve as linker histones. Generally, H1 and H5 are homologous proteins involved in higher-order chromatin structures. But, other histones are involved in the formation of nucleosomes. The dimers of these four core histones form an octameric nucleosome core around which a piece of DNA of about 146 bp wraps 1.65 times, forming a left-handed super-helical turn. The linker histones bind to the nucleosome at the entry and exit site of DNA, locking the DNA in place.  

What is Nonhistone Protein 

Nonhistone protein refers to the proteins remaining in the chromatin structure after histones have been removed. It includes scaffold proteins, Heterochromatin Protein 1, DNA polymerase, and Polycomb, and other motor proteins associated with various functions of DNA. Therefore, these proteins play a key role in the structural support to the DNA, also helping other regulatory functions.  

Figure 2: Role of Nonhistone Proteins in Histone Modifications

Moreover, Huntingtin protein is a type of scaffold protein responsible for DNA damage repair. Furthermore, Heterochromatin Protein 1 is a family of highly conserved proteins found in the nucleus. It plays a role in gene expression through the formation of heterochromatin, transcriptional activation, etc. On the other hand, Polycomb-group proteins is another family of proteins involved in chromatin remodeling in the epigenetic silencing of genes. Also, DNA polymerase is the key enzyme responsible for the synthesis of new DNA. 

Similarities Between Histone and Nonhistone Protein 

• Histone and nonhistone protein are two types of proteins involved in the formation of the chromatin structure of DNA. 
• Their main function is to provide structural support to the DNA. 
• Also, they have functions in cellular mechanism related to DNA. 

Difference Between Histone and Nonhistone Protein 

Definition 

Histone protein refers to a family of basic proteins associated with DNA in the nucleus, condensing it into chromatin, whereas nonhistone protein refers to those proteins that remain after the histones have been removed.  This is the main difference between histone and nonhistone protein.

Types 

The five types of histone proteins are H1 (or H5), H2A, H2B, H3, and H4 while scaffold proteins, heterochromatin protein 1, Polycomb, and DNA polymerase are nonhistone proteins. 

Conservation across Species 

Histones are a highly conserved type of proteins across species while nonhistone proteins are less conserved. This is also a difference between histone and nonhistone protein.

Function 

Another major difference between histone and nonhistone protein is their function. Histone proteins help to package DNA into nucleosomes while nonhistone proteins play a role in functions related to DNA. 

Conclusion 

Histone protein is the major proteins in the chromatin. It is responsible for the package of DNA through the formation of nucleosomes. In contrast, nonhistone protein represents the other proteins in the chromatin structure; those that remain when histones are removed. They have vital functions in the regulation of DNA-related functions. Therefore, the main difference between histone and nonhistone protein is the type and function.   

References:

1. “What Are Histones?” Immunohistochemistry Guide – Creative Diagnostics, Available Here.
2. “Non-Histone Protein.” ScienceDirect, Elsevier, Available Here.

Image Courtesy:

1. “Nucleosome structure” By Richard Wheeler (Zephyris) – English Wikipedia (CC BY-SA 3.0) via Commons Wikimedia  
2. “Histone acetylation and deacetylation” By Annabelle L. Rodd, Katherine Ververis, and Tom C. Karagiannis (CC BY 3.0) via Commons Wikimedia