Spermatozoa leaving the testis are immotile while their epididymal maturation is known to be essential for the attainment of progressive motility, and for the abilities to recognize and fertilize the oocyte [1, 2]. This maturation process is associated with interaction between spermatozoa and epididymal luminal fluid that bring about changes in sperm membrane structure through removal, addition, and reorganization of the lipid bilayer of the plasma membrane of spermatozoa . Studies have shown that each epididymal region has a distinct function governed by region-specific gene expression [4–7]. Protein families with highly segment-specific expression in the epididymis include, for example, defensins, lipocalins, proteases, protease inhibitors, proteins with a disintegrin and metallopeptidase domain (ADAMs) and cysteine-rich secretory proteins (CRISPs) [8–12]. Region-specific expression of members of these families is thought to be, at least partially, responsible for creating the region-specific luminal environment essential for epididymal sperm maturation.
Interestingly, several cysteine-rich secreted epididymal proteins, especially CRISPs, have also been identified in venoms of various reptiles . The disulfide bridges between the conserved cysteine residues are thought to help maintaining the structure of the venom proteins in the hostile environment of the target prey's bloodstream [14, 15]. Similarly, the dehydrated epididymal luminal fluid is a challenging environment for proteins, and, thus, stability provided by disulfide bridges may be essential for maintaining protein function . Based on their predicted structural properties these cysteine-rich proteins may participate in non-specific defense mechanisms against micro-organisms in the epididymal lumen and/or maturation of the spermatozoa via regulating the activities of certain ion channels, although the exact molecular mechanisms responsible for their functions are still poorly known.
Recently two research groups have independently identified a genomic locus from various mammalian species, including human, with several genes encoding predicted secreted proteins with a cysteine-rich TFP/Ly-6/uPAR domain [15, 17]. The gene family was named Anlp (α-neurotoxin-like protein) and Pate by the two groups. The TFP/Ly-6/uPAR domain has 8-10 highly conserved cysteine residues, but their distribution differs from that of cysteine-rich domain of CRISPs . The cysteines of known PATE proteins form two motifs: C[XX]C[X7-8]C[X6]C[X7-8]C and C[X3]C[X15-16]CC[X4-5]CN, where X stands for any amino acid and the superscripts denote their number, and the N stands for asparagine, which is also conserved. Similarly to CRISPs, the TFP/Ly-6/uPAR domain is identified in snake toxins , but the domain is present in certain membrane receptors as well [20, 21]. Furthermore, the domain was found in murine Ly-6 proteins, and is, thus, termed the TFP/Ly-6/uPAR domain . It confers no known enzymatic activity but binds to a wide range of cell surface receptors, ion channels and enzymes [22, 23]. Of PATE proteins mouse (m) PATE-B has been shown to inhibit Ca2+ uptake of spermatozoa , and mPATE-C, mPATE-P and human (h) PATE-B modulate nicotinic acetylcholine receptors (nAChRs) .
Most of the Anlp/Pate family members have been reported to be predominantly expressed in the prostate and the testis, whereas the UniGene entries show high epididymal expression. In the present study we studied the expression of several Pate family genes in the male reproductive organs with a particular focus in the epididymis, and provide evidence of their regulation by androgens and other testicular factors. In addition, by comparing published data of the genes to the current annotations in the Ensembl and NCBI databases, a putative novel member of the family was discovered.