Squid spermiogenesis: molecular characterization of testis-specific pro-protamines

Area of Science:

• * Marine Biology
• * Molecular Biology
• * Evolutionary Biology

Background:

• * Spermiogenesis in cephalopods, like cuttlefish and squid, involves a nuclear protein transition from histones to spermatid-specific proteins (protein T) and finally to sperm protamines (protein Sp).
• * The squid Loligo pealeii exhibits a similar protein transition process.

Purpose of the Study:

• * To elucidate the primary structures of Loligo pealeii protein T variants (T1 and T2) and protamine variant Sp2.
• * To investigate the relationship between protein T2 and protamine Sp2 and the mechanism of their transition.
• * To provide molecular evidence for the taxonomic relationship between Loligo and Sepia.

Main Methods:

• * Sequence analysis of proteins and their fragments.
• * Mass spectrometric analysis to determine molecular masses and phosphorylation states.
• * Identification of specific cleavage sites in protein processing.

Main Results:

• * Two structural variants of Loligo protein T, T1 and T2, were identified with distinct molecular masses and phosphorylation levels.
• * Protein T2 (79 residues) shares complete structural identity in its C-terminal domain with sperm protamine Sp2 (58 residues).
• * The transition from protein T2 to Sp2 involves hydrolytic cleavage at a specific Gly-Arg bond (residues 18-23).

Conclusions:

• * Testis-specific protein T2 is the direct precursor of sperm protamine Sp2 in Loligo pealeii.
• * The processing mechanism involves specific proteolytic cleavage, a common pathway in protein maturation.
• * The findings offer molecular insights into the evolutionary and taxonomic connections within the Cephalopoda class.