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HU Protein

 

Structure

v     Small, basic protein

v     heterotypic dimmer (2 subunits:HUa and HUb)

 

Mass

v     18kDa (each subunit for 9kDa)

Character

v     Heat-stable DNA binding protein

v     Architectural nuclear protein

v     Non-sequence specific DNA binding protein

v     Nucleoid-associated protein

Function

v     Restrain DNA supercoiling

v     Condense the chromosome

v     Introduce negative supercoiling into circular relaxed DNA template with topoisomerase I

v     Assist the action of DnaA protein in the initiation of DNA replication

v     Regulate DNA replication process

 

 

 

 

                              

 

Text Box: The structure of HU protein (ribbon style on left and cylinder style on right) was obtained from the Protein Data Bank (www.rcsb.org/pdb)(13). PDB identifier code for this structure is 1HUE

 

 

 

 

 

 

Text Box: HU protein structure from web site (http://www.nmr.chem.uu.nl/pdb_nmr/hu.html) According to the description of the web site, this image was obtained from BACILLUS STEAROTHERMOPHILUS (PDB entry date 26-MAY-95)(6).

 

 

 

E.coli HU protein was discovered in 1975 by Josette Rouviere-Yaniv and Francois Gros. This discovery was made in an experiment on an extract form E.coli. The extract was separated by affinity chromatography. 

 

HU protein in E. coli is a small, basic heat stable protein which is Nucleoid-associated.  This heterotypic dimmer protein is composed of two subunits; HUa and HUb, which weigh 9kDa each (4,8,10).  The main function of HU protein is to inhibit DNA supercoiling and to regulate DNA replication process (1,10).  Many different characteristics of HU protein help to initiates DNA replication.  These characteristics are nuceloid-assoicated (formed at origin of replication site), histone-like proteins (accelerating open complex formation, serves as a signal in the cell cycle), and architectural protein (interacting with supercoiled ds DNA) (4,10).  The HU protein in E.coli is a DNA binding protein that is bound to double stranded DNA (dsDNA) in the pre-initiation stage, before the primosome binds to the separated single strands (ssDNA). It is believed that the HU protein induces a conformational change in the dsDNA which destablizes it at the origin of replication, OriC. HU protein has also been shown to have a similar effect as histones on dsDNA, when complexed with topoisomerase, causing a localized supercoil. These findings were also described in a study two years earlier by Nicholas Dixon and Arthur Kornberg.

 

Some research scientists hypothesized that in vivo, HU protein is required for proper synchrony of replication initiation (7).  Also, it was suggested that HU protein acts as a negative modulator of seqA expression, which adjusts the replication initiation by avoiding fast re-initiation (9).  However, there is another suggestion that in the mutant cell, other histone-like proteins can replace HU protein in the initiation of replication (11).

 

References

 

1.         Bahloul, A. Boubrik, F., Rouviere-Yaniv, J., Roles of Escherichia coli histone-like protein HU in DNA replication: HU-beta suppresses the thermosensitivity of dnaA46ts. Biochimie, 83 (2001) 219-229.

2.         Balandina, A., Claret, L., Hengge-Aronis, R., Rouviere-Yaniv, J., The Escherichia coli histone-like protein HU regulates rpoS translation. Mol microbial (2001) Feb;39(4):1069-79.

3.                  Broyles, S and Pettijohn, D. (1986) The Journal of Biological Chemistry. 187, pp. 47-60.      

4.                  Coste, F., Hervouet, N., Oberto, J., Zelwer, C., Castaing, B., Crystallization and prelimnary X-ray diffraction analysis of the homodimeric form a2 of the HU protein from Escherichia coli. Acta Cryst. (1999) D55, 1952-1954.

5.         Dixon, N and Kornberg, A. (1984) Procedings of the National Academy of Science. 81, pp. 424-428.

6.         “HU Protein.”  http://www.nmr.chem.uu.nl/pdb_nmr/hu.html (April 15, 2002).

7.         Jaffe, A., Vinella, D., D’ari, R., The Escherichia coli Histone-Like Protein HU Affects DNA Initiation, Chromosome Partitioning via MukB, and Cell Division via MinCDE. J. Bacteriology, June (1997) 3494-3499.

8.         Kamashev, D., Rouviere-Yaniv, J., The histone-like protein HU binds specifically to DNA recombination and repair intermediates.  EMBO J., Vol. 19 No. 23 (2000) 6527-6535,

9.         Lee, H., Kim, HK., Kang, S., Hong, CB., Yim, J., Hwang, DS., Expression of the seqA gene is negatively modulated by the HU protein in Escherichia coli. Mol Gen Genet (2001) Feb; 264(6): 931-5.

10.       Oberto, J., Rouviere-Yaniv, J., Does the parallel evolution pattern between the replication-segregation proteins and HU have a biological significance?, biochimie, 83 (2001) 61-66.

11.       Ogawa, T., Wada, M., Kano, Y., Imamoto, F., Okazaki, T., DNA replication in Escherichia coli mutants that lack protein HU.  J Bacteriol (1989) Oct;171(10):5672-9.

12.       Rouviere-Yaniv, J and Gros, F. (1975) Procedings of the National Academy of Science. 72, pp. 3428-3432.      

13.       Vis, H., Mariani, M., Vorgias, C. E., Wilson, K. S., Kaptein, R., Boelens, R., Solution structure of the HU protein from Bacillus stearothermophilus. J Mol Biol., 254 (1995) 692.

14.       Yasukawa, H., Ozaki, E., Nakahama, K., Masamune, Y., HU protein binding to the replication origin of the rolling-circle plasmid pKYM enhances DNA replication. Mol Gen genet (1997) May 20;254(5):548-54.