posted on 2013-12-06, 00:00authored byJulia Schumacher, Sanja Ramljak, Abdul
R. Asif, Michael Schaffrath, Hans Zischler, Holger Herlyn
We investigated possible
associations between sequence evolution
of mammalian sperm proteins and their phosphorylation status in humans.
As a reference, spermatozoa from three normozoospermic men were analyzed
combining two-dimensional gel electrophoresis, immunoblotting, and
mass spectrometry. We identified 99 sperm proteins (thereof 42 newly
described) and determined the phosphorylation status for most of them.
Sequence evolution was studied across six mammalian species using
nonsynonymous/synonymous rate ratios (dN/dS) and amino acid distances.
Site-specific purifying selection was assessed employing average ratios
of evolutionary rates at phosphorylated versus nonphosphorylated amino
acids (α). According to our data, mammalian sperm proteins do
not show statistically significant sequence conservation difference,
no matter if the human ortholog is a phosphoprotein with or without
tyrosine (Y) phosphorylation. In contrast, overall phosphorylation
of human sperm proteins, i.e., phosphorylation at serine (S), threonine
(T), and/or Y residues, associates with above-average conservation
of sequences. Complementary investigations suggest that numerous protein–protein
interactants constrain sequence evolution of sperm phosphoproteins.
Although our findings reject a special relevance of Y phosphorylation
for sperm functioning, they still indicate that overall phosphorylation
substantially contributes to proper functioning of sperm proteins.
Hence, phosphorylated sperm proteins might be considered as prime
candidates for diagnosis and treatment of reduced male fertility.