The interaction of the histone H1-related protein φ 0 with chromatin

Abstract

Protein φ 0 is a unique protein which is present in the sperm of the sea cucumber, Holothuria tubulosa. It associates with histones, but its physiological role is unknown. From its amino acid composition and sequence, protein φ 0 can be considered as an H1-related protein. In this paper, we have studied its interaction with chicken erythrocyte chromatin particles of different complexity, from core particles to polynucleosomes. Addition of protein φ 0 results in marked chromatin insolubilization. The higher the molecular weight of the chromatin fragment, the lower is the φ 0/nucleosome molar ratio at which precipitation occurs, so that complete insolubilization of polynucleosomes is achieved at a φ 0/nucleosome molar ratio which is identical to that found in mature H. tubulosa spermatozoa. We have also found th at the interaction of protein φ 0 with chromatin is cooperative. These findings contribute to clarification of the peculiar physico-chemical properties shown by H. tubulosa sperm chromatin and the role played by the φ 0 protein.