Abstract
Protamines are small, highly-specialized, arginine-rich, and intrinsically-disordered chromosomal proteins that replace histones during spermiogenesis in many organisms. Previous evidence supports the notion that, in the animal kingdom, these proteins have evolved from a primitive replication-independent histone H1 involved in terminal cell differentiation. Nevertheless, a direct connection between the two families of chromatin proteins is missing. Here, we primarily used electron transfer dissociation MS-based analyses, revealing that the protamines in the sperm of the liverwort Marchantia polymorpha result from post-translational cleavage of three precursor H1 histones. Moreover, we show that the mature protamines are further post-translationally modified by di-aminopropanelation, and previous studies have reported that they condense spermatid chromatin through a process consisting of liquid-phase assembly likely involving spinodal decomposition. Taken together, our results reveal that the interesting evolutionary ancestry of protamines begins with histone H1 in both the animal and plant kingdoms.
Highlights
Protamines are small, highly-specialized, arginine-rich, and intrinsically-disordered chromosomal proteins that replace histones during spermiogenesis in many organisms
The results described below conclusively show that in Marchantia the post-translational processing of three highly-specialized H1 histones of the PL type, which are expressed in antheridiophores, gives rise to the protamines that are found in the sperm of this organism
Coarse electrophoretic fractionation of these proteins revealed a composition enriched in basic amino acids that was important in reaching this conclusion
Summary
Initial evidence for the presence of protamines in Marchantia polymorpha sperm The first evidence of protamines in M. polymorpha comes from a study published in 1978 by Reynolds and Wolfe [24]. The sequences obtained (see Table 1), primarily using ETD, confirmed the arginine-rich protamine nature of these proteins, and they revealed their structural relation to the three PL proteins that were bioinformatically identified above These results conclusively show that M. polymorpha SNBPs are the post-translational cleavage products of three independent, histone H1–related, and protamine-like precursors. Based on the sequence deduced from the c-ion series, the database BLAST search provided the protein from which the observed species originated as well as the final residue in the protein This ion corresponded to an arginine with an unknown modification of 56.0740 Da greater than an unmodified arginine.
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