SQUID SPERMIOGENESIS - MOLECULAR CHARACTERIZATION OF TESTIS-SPECIFIC PRO-PROTAMINES

Cuttlefish spermiogenesis is characterized by a two-step nuclear prote in transition: histones --> spermatid-specific protein (protein T) --> sperm protamine (protein Sp). A similar situation can be observed in another Cephalopod species, the squid Loligo pealeii. The protein T fr om Loligo consists of two structural variants, T1 and T2 (molecular ma sses: 10788 and 10791 Da respectively), phosphorylated to different de grees (2-6 phosphate groups). The primary structures of these two vari ants and of the protamine variant Sp2 were established from sequence a nalysis and mass spectrometric data of the proteins and their fragment s, T1 and T2 are closely related proteins of 79 residues. The complete structural identity of the C-terminal domain (residues 22-79) of prot ein T2 with the sperm protamine Sp2 (molecular mass 8562 Da, 58 residu es) strongly suggests that the testis-specific protein T2 is indeed th e precursor of the protamine. The transition between the precursor pro tein T and protein Sp results from a hydrolytic cleavage similar to th at found in many proteins that are synthesized as precursors. The proc essing mechanism involves the specific cleavage of a Gly-Arg bond in t he sequence Met/Leu(18)-Lys-Gly-Gly-Arg-Arg(23). Furthermore, the stud y provides molecular evidence on the taxonomic relationship between Lo ligo and Sepia.