Cysteine-containing histone H1-like (PL-I) proteins of sperm

We have determined the presence of cysteine in the protein PL-I from the sp erm of the surf clam Spisula solidissima. The existence of cysteine in this histone H1-related protein is responsible for its previously described agg regation behavior. The location of this residue, within the trypsin-resista nt domain of the protein, has been established. We have also shown that cys teine is ubiquitously present in the PL-I proteins from the sperm of other bivalve mollusks but is absent from other PL of smaller molecular mass (PL- II, PL-III, PL-IV). We have also found cysteine to be present in the PL-I f rom a tunicate (Chelysoma productum) but absent in a PL-I from a fish (Mull us baubatus), The possible significance of the unusual occurrence of cystei ne in these histone-H1-related proteins is discussed. (C) 1999 Wiley-Liss, Inc.