Sperm condensation: FTIR used to investigate protamines
Ezine
- Published: Apr 1, 2011
- Author: David Bradley
- Channels: Infrared Spectroscopy
Proteins spiral for controlProtamines are small basic proteins that condense the genetic material, the DNA, in mature sperm helping to form the head of the sperm. They are rich in the amino acid arginine whose residues are distributed in a number of stretches separated by neutral amino acids. The amino acid accounts for between 60 and 80 percent of the protamine. Now, a team in Spain has used Fourier transform infrared spectroscopy to investigate, for the first time, the secondary structure of protamines in sperm nuclei.Free protamines in solution have little structure but in the presence of DNA adopt various conformations through hydrogen bonding and other intramolecular effects. Understanding protamines could have important implications for male fertility and perhaps for ex vivo fertilisation techniques. Humans have two protamines and their primary role is to replace histones late in the haploid phase of sperm production allowing the DNA to be packed much more tightly. The heads of mature sperm contain little or no other protein-based components. Intriguingly the same proteins, when mixed with insulin, slow the onset and increase the duration of insulin action. Protamine is also used as a pharmaceutical agent in cardiopulmonary bypass surgery where it neutralizes the anti-clotting effects of heparin as well as increasing pulmonary artery pressure and decreasing peripheral blood pressure. Alicia Roque, Inma Ponte and Pedro Suau of the Department of Biochemistry and Molecular Biology at Universidad Autónoma de Barcelona explain that characterising the protamine in sperm nuclei is now possible using FTIR spectroscopy for the first time. FTIR is not disturbed by turbidity and light-scattering effects is well suited to such a study. The team has now investigated salmon and squid protamines using this technique. The salmon protamine, known as salmine, is typical of fish protamines. It has just 32 amino acids in total and is 67% arginine. Squid protamine has two components, Sp1 and Sp2 in a ratio of 1 to 3; Sp2 being the most abundant having 58 amino acids of which 79% arginine. The new work reveals alpha-helices and beta-turns in the secondary structure of these protamines but no beta-sheets were found. The team also identified other secondary structures that are not stabilised by hydrogen bonding between amino acid residues. In salmon, alpha-helix accounts for just 20% of the protein while this proportion is about 40% in squid. The beta-turn motifs are present at the same ratio in each (40%). Intriguingly, the non-stabilised structures are present in the reverse proportions to the alpha-helices; about 40% in salmon, 20% in squid. Circular dichroism experiments in trifluoroethanol and structure predictions confirmed the helical potential of each source. Despite these differences both compounds are equally adept at promoting the tight hexagonal packing of DNA in the sperm head. Theoretical supportThe research lends support to one of two theories as to how protamine binds and thence helps pack DNA. In the first theory, protamine binds either to the narrow or wide groove of the DNA double helix. The new study supports a second theory that protamines form alpha-helical structures when bound to DNA. Either way, the high arginine content of protamines act as anchor points to DNA and are key to how these molecules help DNA fold up into a genetically inactive form in the sperm primed to be released at fertilisation. There is evidence that disruption of protamine levels in human sperm production underpins at least one form of male infertility.
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