Most importantly, several amidated peptides that could be derived from putative C. PAM protein and activity have been identified in mating ciliary ectosomes, but not in vegetative ectosomes ( 27). Transcriptomic studies identified mating as a developmental stage during which expression of both PAM and preproGATI is increased ( 35). When nutrient conditions improve, the zygote hatches, releasing haploid meiotic progeny ( 28). The interaction of mating cell cilia triggers a complex intraciliary signaling pathway that leads to loss of gametic cell walls, formation of mating structures, and cell fusion, yielding a quadriciliate cell that ultimately develops into a diploid zygote. Mating ectosomes contain PAM and a variety of amidated products, including one that could be derived from preproGATI, one of the many putative prepropeptides encoded by the C. Ectosome release increases rapidly when minus and plus gametes are mixed ( 21). Starvation triggers a genetically encoded developmental process resulting in the formation of sexual gametes of opposite mating type (termed minus and plus). Under plentiful nutrient conditions, haploid C. Amidation occurs via a two-step reaction catalyzed by the sequential actions of the monooxygenase (peptidylglycine α-hydroxylating monooxygenase PHM) and lyase (peptidyl-α-hydroxyglycine α-amidating lyase PAL) catalytic cores of PAM ( 17).īioactive ectosomes released from the cilia of vegetative cells contain a subtilisin-like endoprotease (VLE1) that degrades the mother cell wall ( 22, 24, 26), but do not contain PAM ( 27). Peptidylglycine α-amidating monooxygenase (PAM), an ancient copper-dependent monooxygenase, catalyzes a late step (amidation) in the biosynthesis of a broad array of peptides, such as mammalian αMSH and vasopressin, the sea urchin sperm attractant resact, as well as numerous invertebrate venom peptide toxins. Posttranslational modification-including disulfide bond formation, N- and O-glycosylation, lipidation, endo- and exo-proteolysis, and peptide amidation-occur as the newly synthesized peptide precursors travel from the ER to the Golgi complex and are packaged for secretion ( 12– 16). Genes encoding potential peptide precursors can be identified based on the presence of an N-terminal presequence, which directs the nascent chain into the lumen of the endoplasmic reticulum (ER), multiple sites subject to cleavage by subtilisin-like prohormone convertases, and multiple sites at which C-terminal amidation could occur ( 11). Secreted peptides are an ancient means of intercellular communication. Analysis of this pathway affords insight into the evolution of peptidergic signaling this will facilitate studies of the secretory functions of metazoan cilia. Extensive posttranslational modification of proGATI confers stability to its amidated product. Mass spectrometric analysis of a potential prohormone convertase and the amidated proGATI-derived products found in cilia and mating ectosomes link endoproteolytic cleavage to ectosome entry. Unlike metazoan propeptides, modeling studies identified stable domains in proGATI. Here we dissect the processing pathway that leads to formation of an amidated peptidergic sexual signal specifically on the ciliary ectosomes of plus gametes. A synthetic peptide (GATI- amide) that could be generated from a 91-kDa peptide precursor (proGATI) serves as a chemotactic modulator, attracting minus gametes while repelling plus gametes. reinhardtii, a green alga, lack the dense core vesicles in which metazoan peptides are processed and stored, we explored the hypothesis that propeptide processing and secretion occur through the regulated release of ciliary ectosomes. The Chlamydomonas reinhardtii genome encodes what appear to be numerous prepropeptides and enzymes homologous to those used to convert metazoan prepropeptides into bioactive peptide products. Peptidergic intercellular communication controls a wide range of physiological and behavioral responses and occurs throughout eukaryotes. Cilia-derived vesicles (ectosomes), formed by outward budding of the ciliary membrane, carry enzymes and other bioactive products this process represents an ancient mode of regulated secretion. Cilia are sensory and secretory organelles that both receive information from the environment and transmit signals.
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