Plethodontid modulating factor, a hypervariable salamander courtship pheromone in the three-finger protein superfamily

Love bites: male frogs (Plectrohyla, Hylidae) use teeth scratching to deliver sodefrin precursor-like factors to females during amplexus

BACKGROUND

Efficient transfer of chemical signals is important for successful mating in many animal species. Multiple evolutionary lineages of animals evolved direct sex pheromone transmission during traumatic mating-the wounding of the partner with specialized devices-which helps to avoid signal loss to the environment. Although such direct transmission modes of so-called allohormone pheromones are well-documented in invertebrates, they are considered rare in vertebrates. Males of several species of the frog genus Plectrohyla (Hylidae, Anura) have elongated teeth and develop swollen lips during the breeding season. Here we investigated the possibility that these structures are used to scratch the females' skin and apply allohormone pheromones during traumatic mating in several Plectrohyla species.

RESULTS

Our behavioural observations revealed that males press their upper jaw onto the females' dorsum during amplexus, leaving small skin scratches with their teeth. Histological examinations of the males' lips identified specialized mucus glands, resembling known amphibian pheromone glands. Whole-transcriptome sequencing of these breeding glands showed high expression of sodefrin precursor-like factor (SPF) proteins, which are known to have a pheromone function in multiple amphibian species.

CONCLUSIONS

Our study suggests SPF delivery via traumatic mating in several anuran species: the males have specialized breeding glands in the lips for production and secretion and use their elongated teeth as wounding devices for application. We hypothesize that these SPF proteins end up in the females' circulatory system, where understanding their exact function will require further molecular, physiological and behavioural testing.

Pheromone Gene Diversification and the Evolution of Courtship Glands in Plethodontid Salamanders

Proteinaceous pheromones that diversify through gene duplication can result in shifts in courtship cocktails that may serve as a mechanism for reproductive isolation. The molecular evolution of pheromones has been extensively studied in salamanders, but how these genes and associated novel courtship glands have codiversified has not been evaluated. In this study we used transcriptional analyses to examine the relationship between pheromone diversification and gland type in three divergent lineages of plethodontid salamanders. Our results revealed that plethodontid salamanders express up to eight divergent Sodefrin Precursor-like Factor genes (spf, representing both alpha and beta subfamilies) along with Plethodontid Modulating Factor (pmf) and Plethodontid Receptivity Factor (prf). Expression of pheromone genes is tissue specific with pmf, prf, and some spf genes restricted to the mental gland. In contrast, the caudal gland shows strong expression of the other spf genes. We found evidence for punctuated changes in pheromone cocktail composition related to the loss of metamorphosis, and subsequent extreme reduction of the mental gland, in a paedomorphic lineage. Our study provides insight into how pheromone diversification can be partitioned into unique glands, which may lead to cocktail specificity in behavioral modules during courtship.

An annual cycle of gene regulation in the red-legged salamander mental gland: from hypertrophy to expression of rapidly evolving pheromones

Background

Cell differentiation is mediated by synchronized waves of coordinated expression for hundreds to thousands of genes, and must be regulated to produce complex tissues and phenotypes. For many animal species, sexual selection has driven the development of elaborate male ornaments, requiring sex-specific differentiation pathways. One such male ornament is the pheromone-producing mental gland of the red-legged salamander (Plethodon shermani). Mental gland development follows an annual cycle of extreme hypertrophy, production of pheromones for the ~ 2 month mating season, and then complete resorption before repeating the process in the following year. At the peak of the mating season, the transcriptional and translational machinery of the mental gland are almost exclusively redirected to the synthesis of rapidly evolving pheromones. Of these pheromones, Plethodontid Modulating Factor (PMF) has experienced an unusual history: following gene duplication, the protein coding sequence diversified from positive sexual selection while the untranslated regions have been conserved by purifying selection. The molecular underpinnings that bridge the processes of gland hypertrophy, pheromone synthesis, and conservation of the untranslated regions remain to be determined.

Results

Using Illumina sequencing, we prepared a de novo transcriptome of the mental gland at six stages of development. Differential expression analysis and immunohistochemistry revealed that the mental gland initially adopts a highly proliferative, almost tumor-like phenotype, followed by a rapid increase in pheromone mRNA and protein. One likely player in this transition is Cold Inducible RNA Binding Protein (CIRBP), which selectively and cooperatively binds the highly conserved PMF 3′ UTR. CIRBP, along with other proteins associated with stress response, have seemingly been co-opted to aid in mental gland development by helping to regulate pheromone synthesis.

Conclusions

The P. shermani mental gland utilizes a complex system of transcriptional and post-transcriptional gene regulation to facilitate its hypertrophication and pheromone synthesis. The data support the evolutionary interplay of coding and noncoding segments in rapid gene evolution, and necessitate the study of co-evolution between pheromone gene products and their transcriptional/translational regulators. Additionally, the mental gland could be a powerful emerging model of regulated tissue proliferation and subsequent resorption within the dermis and share molecular links to skin cancer biology.

Electronic supplementary material

The online version of this article (10.1186/s12861-019-0190-z) contains supplementary material, which is available to authorized users.

Olfactory effects of a hypervariable multicomponent pheromone in the red-legged salamander, Plethodon shermani

Chemical communication via chemosensory signaling is an essential process for promoting and modifying reproductive behavior in many species. During courtship in plethodontid salamanders, males deliver a mixture of non-volatile proteinaceous pheromones that activate chemosensory neurons in the vomeronasal epithelium (VNE) and increase female receptivity. One component of this mixture, Plethodontid Modulating Factor (PMF), is a hypervariable pheromone expressed as more than 30 unique isoforms that differ between individual males-likely driven by co-evolution with female receptors to promote gene duplication and positive selection of the PMF gene complex. Courtship trials with females receiving different PMF isoform mixtures had variable effects on female mating receptivity, with only the most complex mixtures increasing receptivity, such that we believe that sufficient isoform diversity allows males to improve their reproductive success with any female in the mating population. The aim of this study was to test the effects of isoform variability on VNE neuron activation using the agmatine uptake assay. All isoform mixtures activated a similar number of neurons (>200% over background) except for a single purified PMF isoform (+17%). These data further support the hypothesis that PMF isoforms act synergistically in order to regulate female receptivity, and different putative mechanisms are discussed.

Beyond sodefrin: evidence for a multi-component pheromone system in the model newt Cynops pyrrhogaster (Salamandridae)

Sodefrin, a decapeptide isolated from the male dorsal gland of the Japanese fire belly newt Cynops pyrrhogaster, was the first peptide pheromone identified from a vertebrate. The fire belly salamander and sodefrin have become a model for sex pheromone investigation in aquatically courting salamanders ever since. Subsequent studies in other salamanders identified SPF protein courtship pheromones of around 20 kDa belonging to the same gene-family. Although transcripts of these proteins could be PCR-amplified in Cynops, it is currently unknown whether they effectively use full-length SPF pheromones next to sodefrin. Here we combined transcriptomics, proteomics and phylogenetics to investigate SPF pheromone use in Cynops pyrrhogaster. Our data show that not sodefrin transcripts, but multiple SPF transcripts make up the majority of the expression profile in the dorsal gland of this newt. Proteome analyses of water in which a male has been courting confirm that this protein blend is effectively secreted and tail-fanned to the female. By combining phylogenetics and expression data, we show that independent evolutionary lineages of these SPF’s were already expressed in ancestral Cynops species before the origin of sodefrin. Extant Cynops species continue to use this multi-component pheromone system, consisting of various proteins in addition to a lineage-specific peptide.

Proteinaceous Pheromone Homologs Identified from the Cloacal Gland Transcriptome of a Male Axolotl, Ambystoma mexicanum

Pheromones play an important role in modifying vertebrate behavior, especially during courtship and mating. Courtship behavior in urodele amphibians often includes female exposure to secretions from the cloacal gland, as well as other scent glands. The first vertebrate proteinaceous pheromone discovered, the decapeptide sodefrin, is a female attracting pheromone secreted by the cloacal gland of male Cynops pyrrhogaster. Other proteinaceous pheromones in salamanders have been shown to elicit responses from females towards conspecific males. The presence and levels of expression of proteinaceous pheromones have not been identified in the family Ambystomatidae, which includes several important research models. The objective of this research was therefore to identify putative proteinaceous pheromones from male axolotls, Ambystoma mexicanum, as well as their relative expression levels. The results indicate that axolotls possess two different forms of sodefrin precursor-like factor (alpha and beta), as well as a putative ortholog of plethodontid modulating factor. The beta form of sodefrin precursor-like factor was amongst the most highly expressed transcripts within the cloacal gland. The ortholog of plethodontid modulating factor was expressed at a level equivalent to the beta sodefrin precursor-like factor. The results are from a single male axolotl; therefore, we are unable to assess how representative our results may be. Nevertheless, the presence of these highly expressed proteinaceous pheromones suggests that male axolotls use multiple chemical cues to attract female conspecifics. Behavioral assays would indicate whether the putative protein pheromones elicit courtship activity from female axolotls.

Co-option and evolution of non-olfactory proteinaceous pheromones in a terrestrial lungless salamander

Gene co-option is a major force in the evolution of novel biological functions. In plethodontid salamanders, males deliver proteinaceous courtship pheromones to the female olfactory system or transdermally to the bloodstream. Molecular studies identified three families of highly duplicated, rapidly evolving pheromones (PRF, PMF, and SPF). Analyses for Plethodon salamanders revealed pheromone mixtures of primarily PRF and PMF. The current study demonstrates that in Desmognathus ocoee--a plesiomorphic species with transdermal delivery--SPF is the major pheromone component representing >30% of total protein. Chromatographic profiles of D. ocoee pheromones were consistent from May through October. LC/MS-MS analysis suggested uniform SPF isoform expression between individual male D. ocoee. A gene ancestry for SPF with the Three-Finger Protein superfamily was supported by intron-exon boundaries, but not by the disulfide bonding pattern. Further analysis of the pheromone mixture revealed paralogs to peptide hormones that contained mutations in receptor binding regions, such that these novel molecules may alter female physiology by acting as hormone agonists/antagonists. Cumulatively, gene co-option, duplication, and neofunctionalization have permitted recruitment of additional gene families for pheromone activity. Such independent co-option events may be playing a key role in salamander speciation by altering male traits that influence reproductive success.

Chemosignals, hormones, and amphibian reproduction

This article is part of a Special Issue "Chemosignals and Reproduction". Amphibians are often thought of as relatively simple animals especially when compared to mammals. Yet the chemosignaling systems used by amphibians are varied and complex. Amphibian chemosignals are particularly important in reproduction, in both aquatic and terrestrial environments. Chemosignaling is most evident in salamanders and newts, but increasing evidence indicates that chemical communication facilitates reproduction in frogs and toads as well. Reproductive hormones shape the production, dissemination, detection, and responsiveness to chemosignals. A large variety of chemosignals have been identified, ranging from simple, invariant chemosignals to complex, variable blends of chemosignals. Although some chemosignals elicit straightforward responses, others have relatively subtle effects. Review of amphibian chemosignaling reveals a number of issues to be resolved, including: 1) the significance of the complex, individually variable blends of courtship chemosignals found in some salamanders, 2) the behavioral and/or physiological functions of chemosignals found in anuran "breeding glands", 3) the ligands for amphibian V2Rs, especially V2Rs expressed in the main olfactory epithelium, and 4) the mechanism whereby transdermal delivery of chemosignals influences behavior. To date, only a handful of the more than 7000 species of amphibians has been examined. Further study of amphibians should provide additional insight to the role of chemosignals in reproduction.

Expression of venom gene homologs in diverse python tissues suggests a new model for the evolution of snake venom

Snake venom gene evolution has been studied intensively over the past several decades, yet most previous studies have lacked the context of complete snake genomes and the full context of gene expression across diverse snake tissues. We took a novel approach to studying snake venom evolution by leveraging the complete genome of the Burmese python, including information from tissue-specific patterns of gene expression. We identified the orthologs of snake venom genes in the python genome, and conducted detailed analysis of gene expression of these venom homologs to identify patterns that differ between snake venom gene families and all other genes. We found that venom gene homologs in the python are expressed in many different tissues outside of oral glands, which illustrates the pitfalls of using transcriptomic data alone to define "venom toxins." We hypothesize that the python may represent an ancestral state prior to major venom development, which is supported by our finding that the expansion of venom gene families is largely restricted to highly venomous caenophidian snakes. Therefore, the python provides insight into biases in which genes were recruited for snake venom systems. Python venom homologs are generally expressed at lower levels, have higher variance among tissues, and are expressed in fewer organs compared with all other python genes. We propose a model for the evolution of snake venoms in which venom genes are recruited preferentially from genes with particular expression profile characteristics, which facilitate a nearly neutral transition toward specialized venom system expression.

Frog nuptial pads secrete mating season-specific proteins related to salamander pheromones

Males of many frog species develop spiny nuptial pads with underlying glands on their thumbs during the mating period. We used 3D visualization on the European common frog Rana temporaria to show that the morphology of these glands allows the channelling of secreted molecules to the pad's surface during amplexus. Combined transcriptome and proteome analyses show that proteins of the Ly-6/uPAR family, here termed amplexins, are highly expressed in the nuptial glands during the mating season, but are totally absent outside that period. The function of amplexins remains unknown, but it is interesting to note that they share structural similarities with plethodontid modulating factors, proteins that influence courtship duration in salamanders.

Identification and characterization of a taxon-specific three-finger toxin from the venom of the Green Vinesnake (Oxybelis fulgidus; family Colubridae)

Snake venoms contain a variety of protein and peptide toxins, and the three-finger toxins (3FTxs) are among the best characterized family of venom proteins. The compact nature and highly conserved molecular fold of 3FTxs, together with their abundance in many venoms, has contributed to their utility in structure-function studies. Although many target the nicotinic acetylcholine receptor of vertebrate skeletal muscle, often binding with nanomolar Kds, several non-conventional 3FTxs show pronounced taxon-specific neurotoxic effects. Here we describe the purification and characterization of fulgimotoxin, a monomeric 3FTx from the venom of Oxybelis fulgidus, a neotropical rear-fanged snake. Fulgimotoxin retains the canonical 5 disulfides of the non-conventional 3FTxs and is highly neurotoxic to lizards; however, mice are unaffected, demonstrating that this toxin is taxon-specific in its effects. Analysis of structural features of fulgimotoxin and other colubrid venom 3FTxs indicate the presence of a "colubrid toxin motif" (CYTLY) and a second conserved segment (WAVK) found in Boiga and Oxybelis taxon-specific 3FTxs, both in loop II. Because specific residues in loop II conventional α-neurotoxic 3FTxs are intimately associated with receptor binding, we hypothesize that this loop, with its highly conserved substitutions, confers taxon-specific neurotoxicity. These findings underscore the importance of rear-fanged snake venoms for understanding the evolution of toxin molecules and demonstrate that even among well-characterized toxin families, novel structural and functional motifs may be found.

Proteomic and UTR analyses of a rapidly evolving hypervariable family of vertebrate pheromones

During the annual mating season, the mental gland of male plethodontid salamanders diverts its protein synthesizing capacity to the production of courtship pheromones that increase female receptivity. Plethodontid modulating factor (PMF), a highly disulfide-bonded 7-kDa pheromone, shows unusual hypervariability with each male expressing >30 isoforms. Twenty-eight PMFs were purified and matched by proteomic analyses to cDNA sequences. In contrast to coding sequence hypervariability, the untranslated regions (UTRs) show extraordinary conservation, no predicted microRNA binding sites, and an overlapping triplet polyadenylation signal. Full-length cDNA sequencing revealed three PMF gene classes containing subclasses of clustered sequences that support ≥ 13 PMF gene duplications. The unusual phenomena of hypervariable coding regions embedded within extremely conserved UTRs is proposed to occur by a disjunctive evolutionary process. During the short courtship season, the UTRs are hypothesized to subsume and coordinate the transcriptional and translational regulatory mechanisms of the mental gland. PMF, as a secreted protein with limited metabolic feedback in the male, is under minimal mutational restraint and thus has experienced highly accelerated rates of evolution. Consequently, plethodontid salamanders may provide a unique model for furthering our understanding of the selective forces that determine differential rates of gene duplication and evolution in protein families.

Functional convergence of signalling by GPI-anchored and anchorless forms of a salamander protein implicated in limb regeneration

The GPI-anchor is an established determinant of molecular localisation and various functional roles have been attributed to it. The newt GPI-anchored three-finger protein (TFP) Prod1 is an important regulator of cell behaviour during limb regeneration, but it is unclear how it signals to the interior of the cell. Prod1 was expressed by transfection in cultured newt limb cells and activated transcription and expression of matrix metalloproteinase 9 (MMP9) by a pathway involving ligand-independent activation of epidermal growth factor receptor (EGFR) signalling and phosphorylation of extracellular regulated kinase 1 and 2 (ERK1/2). This was dependent on the presence of the GPI-anchor and critical residues in the α-helical region of the protein. Interestingly, Prod1 in the axolotl, a salamander species that also regenerates its limbs, was shown to activate ERK1/2 signalling and MMP9 transcription despite being anchorless, and both newt and axolotl Prod1 co-immunoprecipitated with the newt EGFR after transfection. The substitution of the axolotl helical region activated a secreted, anchorless version of the newt molecule. The activity of the newt molecule cannot therefore depend on a unique property conferred by the anchor. Prod1 is a salamander-specific TFP and its interaction with the phylogenetically conserved EGFR has implications for our view of regeneration as an evolutionary variable.

Pheromonal communication in amphibians

Pheromonal communication is widespread in salamanders and newts and may also be important in some frogs and toads. Several amphibian pheromones have been behaviorally, biochemically and molecularly identified. These pheromones are typically peptides or proteins. Study of pheromone evolution in plethodontid salamanders has revealed that courtship pheromones have been subject to continual evolutionary change, perhaps as a result of co-evolution between the pheromonal ligand and its receptor. Pheromones are detected by the vomeronasal organ and main olfactory epithelium. Chemosensory neurons express vomeronasal receptors or olfactory receptors. Frogs have relatively large numbers of vomeronasal receptors that are transcribed in both the vomeronasal organ and the main olfactory epithelium. Salamander vomeronasal receptors apparently are restricted to the vomeronasal organ. To date, no chemosensory ligands have been matched to vomeronasal receptors or olfactory receptors so it is unknown whether particular receptor types are (1) specialized for detection of pheromones versus other chemosignals, or (2) specialized for detection of volatile, nonvolatile, or water-borne chemosignals. Despite progress in understanding amphibian pheromonal communication, only a small fraction of amphibian species have been examined. Study of additional species of amphibians will indicate which traits related to pheromonal communication are evolutionarily conserved and which traits have diverged over time.

Evidence for the local evolution of mechanisms underlying limb regeneration in salamanders

The most extensive regenerative ability in adult vertebrates is found in the salamanders. Although it is often suggested that regeneration is an ancestral property for vertebrates, our studies on the cell-surface three-finger-protein Prod 1 provide clear evidence for the importance of local evolution of limb regeneration in salamanders. Prod 1 is implicated in both patterning and growth in the regeneration of limbs. It interacts with well-conserved proteins such as the epidermal growth-factor receptor and the anterior gradient protein that are widely expressed in phylogeny. A detailed analysis of the structure and sequence of Prod 1 in relation to other vertebrate three-finger proteins in mammals and zebra fish supports the view that it is a salamander-specific protein. This is the first example of a taxon-specific protein that is clearly implicated in the mechanisms of regeneration. We propose the hypothesis that regeneration depends on the activity of taxon-specific components in orchestrating a cellular machinery that is extensively conserved between regenerating and non-regenerating taxa. This hypothesis has significant implications for our outlook on regeneration in vertebrates, as well as for the strategies employed in extending regenerative ability in mammals.

Pheromone communication in amphibians and reptiles

This selective review considers herpetological papers that feature the use of chemical cues, particularly pheromones involved in reproductive interactions between potential mates. Primary examples include garter snake females that attract males, lacertid lizards and the effects of their femoral gland secretions, aquatic male newts that chemically attract females, and terrestrial salamander males that chemically persuade a female to mate. Each case study spans a number of research approaches (molecular, biochemical, behavioral) and is related to sensory processing and the physiological effects of pheromone delivery. These and related studies show that natural pheromones can be identified, validated with behavioral tests, and incorporated in research on vomeronasal functional response.

Evolutionary shifts in courtship pheromone composition revealed by EST analysis of plethodontid salamander mental glands

Courtship behavior in salamanders of the family Plethodontidae can last more than an hour. During courtship, males use stereotyped behaviors to repeatedly deliver a variety of proteinaceous pheromones to the female. These pheromones are produced and released from a specialized gland on the male's chin (the mental gland). Several pheromone components are well characterized and represented by high frequency transcripts in cDNA pools derived from plethodontid mental glands. However, evolutionary trends in the overall composition of the pheromonal signal are poorly understood. To address this issue, we used random sequencing to survey the pheromone composition of the mental gland in a representative species from each of three distantly related plethodontid genera. We analyzed 856 high-quality expressed sequence tags (ESTs) derived from unamplified primary cDNA libraries constructed from mental glands of Desmognathus ocoee, Eurycea guttolineata, and Plethodon shermani. We found marked differences among these species in the transcript frequency for three previously identified, functional pheromone components: Plethodontid Receptivity Factor (PRF), Sodefrin Precursor-Like Factor (SPF), and Plethodontid Modulating Factor (PMF). In P. shermani mental glands, transcripts predominately encoded PMF (45% of all ESTs) and PRF (15%), with less than 0.5% SPF. In contrast, in D. ocoee and E. guttolineata the proportions were approximately 20% SPF, 5% PMF, and PRF was absent. For both D. ocoee and E. guttolineata, peptide hormone-like transcripts occur at high frequency and may encode peptides that change the physiological state of the female, influencing the female's likelihood to complete courtship. These and previous results indicate that the evolution of courtship pheromones in the Plethodontidae is dynamic, contrasting with the predominant mode of evolutionary stasis for courtship behavior and morphology.

Sexual communication via peptide and protein pheromones

Pheromones are specific substances utilized by various organisms for intraspecific communication about sex, strain, or species. Although pheromones in terrestrial animals tend to be volatile airborne chemicals, large non-volatile molecules such as peptides and proteins are also utilized for sociosexual communication. Peptide pheromones are recognized by specific receptors expressed in the vertebrate vomeronasal organ that comprises a unique chemosensory system. The information is sent to the hypothalamic area wherein the signal is further integrated, leading to various pheromonal outputs. In this review, current knowledge on the structure and function of peptide and protein pheromones in vertebrates as well as the mechanisms underlying receptor-mediated signal processing will be summarized. The present review will also discuss why, from chemical and ecological points of view, peptide pheromones evolved.

The effects of sex on chemosensory communication in a terrestrial salamander (Plethodon shermani)

Although much evidence reveals sexually dimorphic processing of chemosensory cues by the brain, potential sex differences at more peripheral levels of chemoreception are understudied. In plethodontid salamanders, the volume of the vomeronasal organ (VNO) is almost twice as large in males as compared to females, both in absolute and relative size. To determine whether the structural sexual dimorphism in VNO volume is associated with sex differences in other peripheral aspects of chemosensation, we measured sex differences in chemo-investigation and in responsiveness of the VNO to chemosensory cues. Males and females differed in traits influencing stimulus access to VNO chemosensory neurons. Males chemo-investigated ("nose tapped") neutral substrates and substrates moistened with female body rinses more than did females. Compared to females, males had larger narial structures (cirri) associated with the transfer of substrate-borne chemical cues to the lumen of the VNO. These sex differences in chemo-investigation and narial morphology likely represent important mechanisms for regulating sex differences in chemical communication. In contrast, males and females did not differ in responsiveness of VNO chemosensory neurons to male mental gland extract or female skin secretions. This important result indicates that although males have a substantially larger VNO compared to females, the male VNO was not more responsive to every chemosensory cue that is detected by the VNO. Future studies will determine whether the male VNO is specialized to detect a subset of chemosensory cues, such as female body rinses or female scent marks.

Aplysia temptin - the 'glue' in the water-borne attractin pheromone complex

Temptin, a component of the complex of water-borne protein pheromones that stimulate attraction and mating behavior in the marine mollusk Aplysia, has sequence homology to the epidermal growth factor (EGF)-like domains of higher organisms that mediate protein-cell surface contact during fertilization and blood coagulation. In this work, recombinant temptin for structural and functional studies was produced in Escherichia coli using a cold shock promoter and purified by RP-HPLC. CD spectra confirmed a predominantly beta-sheet structure. Two disulfide bonds were determined via limited proteolysis and MS. One internal disulfide (Cys57-Cys77) was predicted from initial alignments with class I EGF-like domains; the second, between Cys18 and Cys103, could protect temptin against proteolysis in seawater and stabilize its interacting surface. A three-dimensional model of temptin was prepared with our MPACK suite, based on the Ca(2+)-binding, EGF-like domain of the extracellular matrix protein fibrillin. Two temptin residues, Trp52 and Trp79, which align with cysteine residues conserved in fibrillins, lie adjacent to and could stabilize the disulfide bonds and a proposed metal-binding loop. The water-borne pheromone attractin in egg cordon eluates is complexed with other proteins. Docking results with our model and the NMR structure of attractin suggest that one face of temptin interacts with the pheromone, perhaps controlling its access to the cellular receptors. Gel shifts confirmed that temptin complexes with wild-type attractin. These results indicate that temptin, analogous to the role of fibrillin in controlling transforming growth factor-beta concentration, modulates pheromone signaling by direct binding to attractin.