Molecular characterization of six intermediate proteins in the processing of mouse protamine P2 precursor

In mouse spermatozoa, DNA is compacted by two protamines mP1 and mP2. Protamine mP2 (63 residues) is synthesized in spermatid nuclei as a precursor pmP2 (106 residues) which is subsequently processed at the end of spermiogenesis [Yelick, P.C., Balhorn, R., Johnson, P.A., Corzett, M., Mazrimas, J.A., Kleene, K.C. & Hecht, N.B. (1987) Mol. Cell. Biol. 7, 2173-2179]. Six proteins, three of which were described earlier [Chauvière, M., Martinage, A., Debarle, M., Alimi, E., Sautière, P. & Chevaillier, Ph. (1991) C.R. Acad. Sci. 313, 107-112], have molecular and electrophoretic properties similar to those of pmP2. They were isolated from purified testis nuclei and characterized by amino acid composition, N-terminal sequence and peptide mapping. From the amino acid compositions, it appears that all six proteins are rich in arginine, cysteine and histidine and are closely related to pmP2 and mP2. The N-terminal sequence of each protein overlaps a distinct region of the N-terminal part of pmP2. The C-terminal part of protamine mP2 starting at arginine 15 is common to all proteins as assessed by amino acid compositions and peptide maps. All these structural data demonstrate that the six isolated proteins are products of pmP2 precursor processing. The six intermediate proteins pmP2/5, pmP2/11, pmP2/16, pmP2/20, pmP2/26 and pmP2/32 which contain 102, 96, 91, 87, 81 and 75 residues, respectively, are generated from the pmP2 precursor after N-terminal excision of 4, 10, 15, 19, 25 and 31 residues, respectively. The C-terminal sequence of protamine mP2 is strictly identical to that of its precursor; therefore, no maturation occurs in this part of the molecule. At the present time, the proteolytic pathway involved in the amino-terminal processing leading to the mature form of the protamine mP2 (63 residues) has not been elucidated. However, the different representation of six intermediates in the testis suggests that some stages of processing are faster than others or that some cleavage sites are preferred. The proteins described in this paper could result either from stepwise excision of N-terminal residues or from non-sequential cleavages.