Abstract
Sexual selection can explain the rapid evolution of fertilization proteins, yet sperm proteins evolve rapidly even if not directly involved in fertilization. In the marine mollusk abalone, sperm secrete enormous quantities of two rapidly evolving proteins, lysin and sp18, that are stored at nearly molar concentrations. We demonstrate that this extraordinary packaging is achieved by associating into Fuzzy Interacting Transient Zwitterion (FITZ) complexes upon binding the intrinsically disordered FITZ Anionic Partner (FITZAP). FITZ complexes form at intracellular ionic strengths and, upon exocytosis into seawater, lysin and sp18 are dispersed to drive fertilization. NMR analyses revealed that lysin uses a common molecular interface to bind both FITZAP and its egg receptor VERL. As sexual selection alters the lysin-VERL interface, FITZAP coevolves rapidly to maintain lysin binding. FITZAP-lysin interactions exhibit a similar species-specificity as lysin-VERL interactions. Thus, tethered molecular arms races driven by sexual selection can generally explain rapid sperm protein evolution.
Highlights
Genes associated with fertilization are often the fastest evolving in any genome (Swanson and Vacquier, 2002), and in mammals, spermatozoa-specific genes show the greatest divergence between species (Torgerson et al, 2002)
We demonstrate that the rapid evolution of sp6 is due to intra-sperm protein coevolution with lysin and sp18 to allow for their dense storage in the acrosome via novel Fuzzy Interacting Transient Zwitterion (FITZ) complexes
Each isoform was purified using strong anion exchange (SAX) chromatography and reverse-phase high-performance liquid chromatography (RP-HPLC), with mass spectrometry revealing that both isoforms were smaller than their cDNA open reading frame predicted (~3–4 kDa vs ~6 kDa)
Summary
Genes associated with fertilization are often the fastest evolving in any genome (Swanson and Vacquier, 2002), and in mammals, spermatozoa-specific genes show the greatest divergence between species (Torgerson et al, 2002). The marine mollusk abalone is a classic system to study molecular barriers to hybridization (Lewis et al, 1982) and is the source of the first discovered pair of interacting reproductive proteins: sperm lysin and the egg vitelline envelope receptor of lysin (VERL) (Swanson and Vacquier, 1997). While lysin and sp are highly positively charged proteins (+12 to +24), isoforms of sp are highly anionic (À6 to À16) and include an N-terminal polyaspartate region of variable length (1–11 residues) Given this charge complementarity, we hypothesized that sp may facilitate packaging of lysin and sp inside the sperm acrosome. In light of its newly identified function, we have named sp the FITZ Anionic Partner (FITZAP)
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