Multiple sex pheromones and receptors of a mushroom-producing fungus elicit mating in yeast.
The mushroom-producing fungus Schizophyllum commune has thousands of mating types defined, in part, by numerous lipopeptide pheromones and their G protein-linked receptors. Compatible combinations of pheromones and receptors encoded by different mating types regulate a pathway of sexual development leading to mushroom formation and meiosis. A complex set of pheromone-receptor interactions maximizes the likelihood of outbreeding; for example, a single pheromone can activate more than one receptor and a single receptor can be activated by more than one pheromone. The current study demonstrates that the sex pheromones and receptors of Schizophyllum, when expressed in Saccharomyces cerevisiae, can substitute for endogenous pheromone and receptor and induce the yeast pheromone response pathway through the yeast G protein. Secretion of active Schizophyllum pheromone requires some, but not all, of the biosynthetic machinery used by the yeast lipopeptide pheromone a-factor. The specificity of interaction among pheromone-receptor pairs in Schizophyllum was reproduced in yeast, thus providing a powerful system for exploring molecular aspects of pheromone-receptor interactions for a class of seven-transmembrane-domain receptors common to a wide range of organisms. (+info)
Feedback phosphorylation of the yeast a-factor receptor requires activation of the downstream signaling pathway from G protein through mitogen-activated protein kinase.
The two yeast pheromone receptors, the a and alpha-factor receptors, share many functional similarities: both G protein-coupled receptors couple to the same downstream signal transduction pathway, and both receptors undergo feedback regulation involving increased phosphorylation on their C-terminal domains in response to ligand challenge. The present work, which focuses on the signaling mechanism controlling this feedback phosphorylation, indicates one striking difference. While the alpha-factor-induced phosphorylation of the alpha-factor receptor does not require activation of the downstream G protein-directed signaling pathway (B. Zanolari, S. Raths, B. Singer-Kruger, and H. Riezman, Cell 71:755-763, 1992), the a-factor-induced phosphorylation of the a-factor receptor (Ste3p) clearly does. Induced Ste3p phosphorylation was blocked in cells with disruptions of various components of the pheromone response pathway, indicating a requirement of pathway components extending from the G protein down through the mitogen-activated protein kinase (MAPK). Furthermore, Ste3p phosphorylation can be induced in the absence of the a-factor ligand when the signaling pathway is artificially activated, indicating that the liganded receptor is not required as a substrate for induced phosphorylation. While the activation of signaling is critical for the feedback phosphorylation of Ste3p, pheromone-induced gene transcription, one of the major outcomes of pheromone signaling, appears not to be required. This conclusion is indicated by three results. First, ste12Delta cells differ from cells with disruptions of the upstream signaling elements (e.g., ste4Delta, ste20Delta, ste5Delta, ste11Delta, ste7Delta, or fus3Delta kss1Delta cells) in that they clearly retain some capacity for inducing Ste3p phosphorylation. Second, while activated alleles of STE11 and STE12 induce a strong transcriptional response, they fail to induce a-factor receptor phosphorylation. Third, blocking of new pheromone-induced protein synthesis with cycloheximide fails to block phosphorylation. These findings are discussed within the context of a recently proposed model for pheromone signaling (P. M. Pryciak and F. A. Huntress, Genes Dev. 12:2684-2697, 1998): a key step of this model is the activation of the MAPK Fus3p through the G(betagamma)-dependent relocalization of the Ste5p-MAPK cascade to the plasma membrane. Ste3p phosphorylation may involve activated MAPK Fus3p feeding back upon plasma membrane targets. (+info)
An endosome-to-plasma membrane pathway involved in trafficking of a mutant plasma membrane ATPase in yeast.
The plasma membrane ATPase, encoded by PMA1, is delivered to the cell surface via the secretory pathway. Previously, we characterized a temperature-sensitive pma1 mutant in which newly synthesized Pma1-7 is not delivered to the plasma membrane but is mislocalized instead to the vacuole at 37 degrees C. Several vps mutants, which are defective in vacuolar protein sorting, suppress targeting-defective pma1 by allowing mutant Pma1 to move once again to the plasma membrane. In this study, we have analyzed trafficking in the endosomal system by monitoring the movement of Pma1-7 in vps36, vps1, and vps8 mutants. Upon induction of expression, mutant Pma1 accumulates in the prevacuolar compartment in vps36 cells. After chase, a fraction of newly synthesized Pma1-7 is delivered to the plasma membrane. In both vps1 and vps8 cells, newly synthesized mutant Pma1 appears in small punctate structures before arrival at the cell surface. Nevertheless, biosynthetic membrane traffic appears to follow different routes in vps8 and vps1: the vacuolar protein-sorting receptor Vps10p is stable in vps8 but not in vps1. Furthermore, a defect in endocytic delivery to the vacuole was revealed in vps8 (and vps36) but not vps1 by endocytosis of the bulk membrane marker FM 4-64. Moreover, in vps8 cells, there is defective down-regulation from the cell surface of the mating receptor Ste3, consistent with persistent receptor recycling from an endosomal compartment to the plasma membrane. These data support a model in which mutant Pma1 is diverted from the Golgi to the surface in vps1 cells. We hypothesize that in vps8 and vps36, in contrast to vps1, mutant Pma1 moves to the surface via endosomal intermediates, implicating an endosome-to-surface traffic pathway. (+info)
VPS21 controls entry of endocytosed and biosynthetic proteins into the yeast prevacuolar compartment.
Mutations in the VPS (vacuolar protein sorting) genes of Saccharomyces cerevisiae have been used to define the trafficking steps that soluble vacuolar hydrolases take en route from the late Golgi to the vacuole. The class D VPS genes include VPS21, PEP12, and VPS45, which appear to encode components of a membrane fusion complex involved in Golgi-to-endosome transport. Vps21p is a member of the Rab family of small Ras-like GTPases and shows strong homology to the mammalian Rab5 protein, which is involved in endocytosis and the homotypic fusion of early endosomes. Although Rab5 and Vps21p appear homologous at the sequence level, it has not been clear if the functions of these two Rabs are similar. We find that Vps21p is an endosomal protein that is involved in the delivery of vacuolar and endocytosed proteins to the vacuole. Vacuolar and endocytosed proteins accumulate in distinct transport intermediates in cells that lack Vps21p function. Therefore, it appears that Vps21p is involved in two trafficking steps into the prevacuolar/late endosomal compartment. (+info)
Ubiquitination of the PEST-like endocytosis signal of the yeast a-factor receptor.
A 58-residue-long, PEST-like sequence within the yeast a-factor receptor (Ste3p) specifies the ubiquitination, endocytosis, and consequent vacuolar degradation of the receptor protein (Roth, A. F., Sullivan, D. M., and Davis, N. G. (1998) J. Cell Biol. 142, 949-961). The present work investigates three lysyl residues that map within this sequence as the potential ubiquitin acceptor sites. Lys --> Arg substitution mutants were tested for effects on both ubiquitination and endocytosis. Results indicate that the three lysines function redundantly; a severe blockade to both ubiquitination and endocytosis is seen only for receptors having all three lysines replaced. Of the three, Lys(432) plays the predominant role; ubiquitination and turnover are significantly impaired for receptors having just the K432R mutation. CNBr fragmentation of the receptor protein, used for the physical mapping of the ubiquitin attachment sites, showed PEST-like sequence lysines to be modified both with single ubiquitin moieties as well with short multi-ubiquitin chains, two or three ubiquitins long. Thus, in addition to being the signal for ubiquitination, the Ste3p PEST-like sequence also provides the site for ubiquitin attachment. To test if this endocytosis signal functions solely for ubiquitination, we have asked if the requirement for the PEST-like sequence in endocytosis might be bypassed through pre-attachment of ubiquitin to the receptor protein. Indeed, Ste3-ubiquitin translational fusions that have a ubiquitin moiety fused to the receptor in place of the PEST-like signal do undergo rapid endocytosis and vacuolar turnover. We conclude that ubiquitin alone, with no required contribution from receptor sequences, provides the sufficient signal for initiating uptake. In addition, our results confirm conclusions originally drawn from studies with the alpha-factor receptor (Terrell, J., Shih, S., Dunn, R., and Hicke, L. (1998) Mol. Cell 1, 193-202), namely that mono-ubiquitin, and not multi-ubiquitin chains provide the primary recognition determinant for uptake. Although mono-ubiquitination suffices, our results indicate that multi-ubiquitination serves to augment the rate of uptake. (+info)
Akr1p and the type I casein kinases act prior to the ubiquitination step of yeast endocytosis: Akr1p is required for kinase localization to the plasma membrane.
Ubiquitination of the plasma membrane-localized yeast a-factor receptor (Ste3p) triggers a rapid, ligand-independent endocytosis leading to its vacuolar degradation. This report identifies two mutants that block uptake by blocking ubiquitination, these being mutant either for the ankyrin repeat protein Akr1p or for the redundant type I casein kinases Yck1p and Yck2p. While no obvious defect was seen for wild-type Ste3p phosphorylation in akr1 or yck mutant backgrounds, examination of the Delta320-413 Ste3p deletion mutant phosphorylation did reveal a clear defect in both mutants. The Delta320-413 deletion removes 18 Ser-Thr residues (possible YCK-independent phosphorylation sites) yet retains the 15 Ser-Thr residues of the Ste3p PEST-like ubiquitination-endocytosis signal. Two other phenotypes link akr1 and yck mutants: both are defective in phosphorylation of wild-type alpha-factor receptor, and while both are defective for Ste3p constitutive internalization, both remain partially competent for the Ste3p ligand-dependent uptake mode. Yck1p-Yck2p may be the function responsible in phosphorylation of the PEST-like ubiquitination-endocytosis signal. Akr1p appears to function in localizing Yck1p-Yck2p to the plasma membrane, a localization that depends on prenylation of C-terminal dicysteinyl motifs. In akr1Delta cells, Yck2p is mislocalized, showing a diffuse cytoplasmic localization identical to that seen for a Yck2p mutant that lacks the C-terminal Cys-Cys, indicating a likely Akr1p requirement for the lipid modification of Yck2p, for prenylation, or possibly for palmitoylation. (+info)
Recycling of the yeast a-factor receptor.
The yeast a-factor receptor (Ste3p) is subject to two mechanistically distinct modes of endocytosis: a constitutive, ligand-independent pathway and a ligand-dependent uptake pathway. Whereas the constitutive pathway leads to degradation of the receptor in the vacuole, the present work finds that receptor internalized via the ligand-dependent pathway recycles. With the a-factor ligand continuously present in the culture medium, trafficking of the receptor achieves an equilibrium in which continuing uptake to endosomal compartments is balanced by its recycling return to the plasma membrane. Withdrawal of ligand from the medium leads to a net return of the internalized receptor back to the plasma membrane. Although recycling is demonstrated for receptors that lack the signal for constitutive endocytosis, evidence is provided indicating a participation of recycling in wild-type Ste3p trafficking as well: a-factor treatment both slows wild-type receptor turnover and results in receptor redistribution to intracellular endosomal compartments. Apparently, a-factor acts as a switch, diverting receptor from vacuole-directed endocytosis and degradation, to recycling. A model is presented for how the two Ste3p endocytic modes may collaborate to generate the polarized receptor distribution characteristic of mating cells. (+info)
Asg7p-Ste3p inhibition of pheromone signaling: regulation of the zygotic transition to vegetative growth.
The inappropriate expression of the a-factor pheromone receptor (Ste3p) in the MATa cell leads to a striking inhibition of the yeast pheromone response, the result of a functional interaction between Ste3p and some MATa-specific protein. The present work identifies this protein as Asg7p. Normally, expression of Ste3p and Asg7p is limited to distinct haploid mating types, Ste3p to MATalpha cells and Asg7p to MATa cells. Artificial coexpression of the two in the same cell, either a or alpha, leads to dramatic inhibition of the pheromone response. Ste3p-Asg7p coexpression also perturbs the membrane trafficking of Ste3p: Ste3p turnover is slowed, a result of an Asg7p-mediated retardation of the secretory delivery of the newly synthesized receptor to the plasma membrane. However, in the absence of ectopic Ste3p expression, the asg7Delta mutation is without consequence either for pheromone signaling or overall mating efficiency of a cells. Indeed, the sole phenotype that can be assigned to MATa asg7Delta cells is observed following zygotic fusion to its alpha mating partner. Though formed at wild-type efficiency, zygotes from these pairings are morphologically abnormal. The pattern of growth is deranged: emergence of the first mitotic bud is delayed, and, in its place, growth is apparently diverted into a novel structure superficially resembling the polarized mating projection characteristic of haploid cells responding to pheromone. Together these results suggest a mechanism in which, following the zygotic fusion event, Ste3p and Asg7p gain access to one another and together act to repress the pheromone response, promoting the transition of the new diploid cell to vegetative growth. (+info)