Subunit structures of different electrophoretic forms of nucleosomes

Area of Science:

• Chromatin biology
• Molecular genetics
• Biochemistry

Background:

• Micrococcal nuclease digestion of mammalian chromatin yields five electrophoretic mononucleosome classes (MI-MV).
• These classes exhibit internal heterogeneity due to minor protein components.

Purpose of the Study:

• To elucidate the subunit composition of different mononucleosome classes.
• To investigate the roles of histone H1 and non-histone proteins (HMG-17, HMG-14) in mononucleosome structure.

Main Methods:

• Chromatin reconstitution using stripped nucleosomes, histone H1, and non-histone proteins (HMG-17/HMG-14).
• One- and two-dimensional electrophoretic analyses of native and reconstituted mononucleosomes.

Main Results:

• Reconstitution successfully generated the five native mononucleosome electrophoretic forms.
• Specific subunit compositions were determined: MI (core octamer), MII (octamer + HMG-17), MIIIA (octamer + H1), MIIIB (octamer + 2xHMG-17), MIV (octamer + H1 + HMG-17), MV (octamer + H1 + 2xHMG-17).
• HMG-14 can substitute for HMG-17, and mononucleosomes have binding sites for at least one H1 and two non-histone proteins.

Conclusions:

• Mononucleosome heterogeneity arises from variations in associated histone H1 and non-histone proteins.
• Independent binding sites exist for histone H1 and non-histone chromosomal proteins on mononucleosomes.
• HMG-17 exhibits dynamic binding site interchangeability.