UV-A/B radiation rapidly activates photoprotective mechanisms in Chlamydomonas reinhardtii

Conversion of light energy into chemical energy through photosynthesis in the chloroplasts of photosynthetic organisms is essential for photoautotrophic growth, and non-photochemical quenching (NPQ) of excess light energy prevents the generation of reactive oxygen species and maintains efficient photosynthesis under high light. In the unicellular green alga Chlamydomonas reinhardtii, NPQ is activated as a photoprotective mechanism through wavelength-specific light signaling pathways mediated by the phototropin (blue light) and ultra-violet (UV) light photoreceptors, but the biological significance of photoprotection activation by light with different qualities remains poorly understood. Here, we demonstrate that NPQ-dependent photoprotection is activated more rapidly by UV than by visible light. We found that induction of gene expression and protein accumulation related to photoprotection was significantly faster and greater in magnitude under UV treatment compared with that under blue- or red-light treatment. Furthermore, the action spectrum of UV-dependent induction of photoprotective factors implied that C. reinhardtii senses relatively long-wavelength UV (including UV-A/B), whereas the model dicot plant Arabidopsis (Arabidopsis thaliana) preferentially senses relatively short-wavelength UV (mainly UV-B/C) for induction of photoprotective responses. Therefore, we hypothesize that C. reinhardtii developed a UV response distinct from that of land plants.

Isolation of photoprotective signal transduction mutants by systematic bioluminescence screening in Chlamydomonas reinhardtii

In photosynthetic organisms, photoprotection to avoid overexcitation of photosystems is a prerequisite for survival. Green algae have evolved light-inducible photoprotective mechanisms mediated by genes such as light-harvesting complex stress-related (LHCSR). Studies on the light-dependent regulation of LHCSR expression in the green alga Chlamydomonas reinhardtii have revealed that photoreceptors for blue light (phototropin) and ultraviolet light perception (UVR8) play key roles in initiating photoprotective signal transduction. Although initial light perception via phototropin or UVR8 is known to result in increased LHCSR3 and LHCSR1 gene expression, respectively, the mechanisms of signal transduction from the input (light perception) to the output (gene expression) remain unclear. In this study, to further elucidate the signal transduction pathway of the photoprotective response of green algae, we established a systematic screening protocol for UV-inducible LHCSR1 gene expression mutants using a bioluminescence reporter assay. Following random mutagenesis screening, we succeeded in isolating mutants deficient in LHCSR1 gene and protein expression after UV illumination. Further characterization revealed that the obtained mutants could be separated into 3 different phenotype groups, the “UV-specific”, “LHCSR1-promoter/transcript-specific” and “general photoprotective” mutant groups, which provided further insight into photoprotective signal transduction in C. reinhardtii.

Algal photoprotection is regulated by the E3 ligase CUL4-DDB1DET1

Light is essential for photosynthesis, but the amounts of light that exceed an organism's assimilation capacity can cause serious damage¹. Photosynthetic organisms minimize such potential harm through protection mechanisms collectively referred to as non-photochemical quenching². One mechanism of non-photochemical quenching called energy-dependent quenching (qE quenching) is readily activated under high-light conditions and dissipates excess energy as heat. LIGHT-HARVESTING COMPLEX STRESS-RELATED PROTEINS 1 and 3 (LHCSR1 and LHCSR3) have been proposed to mediate qE quenching in the green alga Chlamydomonas reinhardtii when grown under high-light conditions³. LHCSR3 induction requires a blue-light photoreceptor, PHOTOTROPIN (PHOT)⁴, although the signal transduction pathway between PHOT and LHCSR3 is not yet clear. Here, we identify two phot suppressor loci involved in qE quenching: de-etiolated 1 (det1)⁵ and damaged DNA-binding 1 (ddb1)⁶. Using a yeast two-hybrid analysis and an inhibitor assay, we determined that these two genetic elements are part of a protein complex containing CULLIN 4 (CUL4). These findings suggest a photoprotective role for the putative E3 ubiquitin ligase CUL4-DDB1^DET1 in unicellular photosynthetic organisms that may mediate blue-light signals to LHCSR1 and LHCSR3 gene expression.

Phospholipid flippases and Sfk1p, a novel regulator of phospholipid asymmetry, contribute to low permeability of the plasma membrane

Phospholipid flippase (type 4 P-type ATPase) plays a major role in the generation of phospholipid asymmetry in eukaryotic cell membranes. Loss of Lem3p-Dnf1/2p flippases leads to the exposure of phosphatidylserine (PS) and phosphatidylethanolamine (PE) on the cell surface in yeast, resulting in sensitivity to PS- or PE-binding peptides. We isolated Sfk1p, a conserved membrane protein in the TMEM150/FRAG1/DRAM family, as a multicopy suppressor of this sensitivity. Overexpression of SFK1 decreased PS/PE exposure in lem3Δ mutant cells. Consistent with this, lem3Δ sfk1Δ double mutant cells exposed more PS/PE than the lem3Δ mutant. Sfk1p was previously implicated in the regulation of the phosphatidylinositol-4 kinase Stt4p, but the effect of Sfk1p on PS/PE exposure in lem3Δ was independent of Stt4p. Surprisingly, Sfk1p did not facilitate phospholipid flipping but instead repressed it, even under ATP-depleted conditions. We propose that Sfk1p negatively regulates transbilayer movement of phospholipids irrespective of directions. In addition, we showed that the permeability of the plasma membrane was dramatically elevated in the lem3Δ sfk1Δ double mutant in comparison with the corresponding single mutants. Interestingly, total ergosterol was decreased in the lem3Δ sfk1Δ mutant. Our results suggest that phospholipid asymmetry is required for the maintenance of low plasma membrane permeability.

Asymmetric distribution of phosphatidylserine is generated in the absence of phospholipid flippases in Saccharomyces cerevisiae

In eukaryotic cells, phosphatidylserine (PS) is predominantly located in the cytosolic leaflet of the plasma membrane; this asymmetry is generated by an unknown mechanism. In this study, we used the PS-specific probe mRFP-Lact-C2 to investigate the possible involvement of type 4 P-type ATPases, also called phospholipid flippases, in the generation of this asymmetry in Saccharomyces cerevisiae. PS was not found in the trans-Golgi Network in wild-type cells, but it became exposed when vesicle formation was compromised in the sec7 mutant, and it was also exposed on secretory vesicles (SVs), as reported previously. However, flippase mutations did not reduce the exposure of PS in either case, even at low levels that would only be detectable by quantitative analysis of mRFP-Lact-C2 fluorescence in isolated SVs. Furthermore, no reduction in the PS level was observed in a mutant with multiple flippase mutations. Because PS was not exposed in a mutant that accumulates ER or cis/medial-Golgi membranes, Golgi maturation seems to be a prerequisite for PS translocation. Our results suggest that an unknown mechanism, possibly a protein with flippase-like activity, acts in conjunction with known flippases to regulate PS translocation.

Interaction of the phospholipid flippase Drs2p with the F-box protein Rcy1p plays an important role in early endosome to trans-Golgi network vesicle transport in yeast

Phospholipid composition of biological membranes differs between the cytoplasmic and exoplasmic leaflets. The type 4 P-type ATPases are phospholipid flippases that generate such membrane phospholipid asymmetry. Drs2p, a flippase in budding yeast, is involved in the endocytic recycling pathway. Drs2p is implicated in clathrin-coated vesicle formation, but the underlying mechanisms are not clearly understood. Here we show that the carboxyl-terminal cytoplasmic region of Drs2p directly binds to Rcy1p, an F-box protein that is also required for endocytic recycling. The Drs2p-binding region was mapped to the amino acids 574-778 region of Rcy1p and a mutant Rcy1p lacking this region was defective in endocytic recycling of a v-SNARE Snc1p. We isolated Drs2p point mutants that reduced the interaction with Rcy1p. The mutation sites were clustered within a small region (a.a. 1260-1268) of Drs2p. Although these point mutants did not exhibit clear phenotypes, combination of them resulted in cold-sensitive growth, defects in endocytic recycling of Snc1p and defective localization of Rcy1p to endosomal membranes like the drs2 null mutant. These results suggest that the interaction of Drs2p with Rcy1p plays an important role for Drs2p function in the endocytic recycling pathway.

Functions of phospholipid flippases

Asymmetrical distribution of phospholipids is generally observed in the eukaryotic plasma membrane. Maintenance and changes of this phospholipid asymmetry are regulated by ATP-driven phospholipid translocases. Accumulating evidence indicates that type 4 P-type ATPases (P4-ATPases, also called flippases) translocate phospholipids from the exoplasmic leaflet to the cytoplasmic leaflet of the plasma membrane and internal membranes. Among P-type ATPases, P4-ATPases are unique in that they are associated with a conserved membrane protein of the Cdc50 family as a non-catalytic subunit. Recent studies indicate that flippases are involved in various cellular functions, including transport vesicle formation and cell polarity. In this review, we will focus on the functional aspect of phospholipid flippases.

Transbilayer phospholipid flipping regulates Cdc42p signaling during polarized cell growth via Rga GTPase-activating proteins

An important problem in polarized morphogenesis is how polarized transport of membrane vesicles is spatiotemporally regulated. Here, we report that a local change in the transbilayer phospholipid distribution of the plasma membrane regulates the axis of polarized growth. Type 4 P-type ATPases Lem3p-Dnf1p and -Dnf2p are putative heteromeric phospholipid flippases in budding yeast that are localized to polarized sites on the plasma membrane. The lem3Delta mutant exhibits prolonged apical growth due to a defect in the switch to isotropic bud growth. In lem3Delta cells, the small GTPase Cdc42p remains polarized at the bud tip where phosphatidylethanolamine remains exposed on the outer leaflet. Intriguingly, phosphatidylethanolamine and phosphatidylserine stimulate GTPase-activating protein (GAP) activity of Rga1p and Rga2p toward Cdc42p, whereas PI(4,5)P(2) inhibits it. We propose that a redistribution of phospholipids to the inner leaflet of the plasma membrane triggers the dispersal of Cdc42p from the apical growth site, through activation of GAPs.

Endocytic recycling in yeast is regulated by putative phospholipid translocases and the Ypt31p/32p-Rcy1p pathway

Phospholipid translocases (PLTs) have been implicated in the generation of phospholipid asymmetry in membrane bilayers. In budding yeast, putative PLTs are encoded by the DRS2 gene family of type 4 P-type ATPases. The homologous proteins Cdc50p, Lem3p, and Crf1p are potential noncatalytic subunits of Drs2p, Dnf1p and Dnf2p, and Dnf3p, respectively; these putative heteromeric PLTs share an essential function for cell growth. We constructed temperature-sensitive mutants of CDC50 in the lem3Delta crf1Delta background (cdc50-ts mutants). Screening for multicopy suppressors of cdc50-ts identified YPT31/32, two genes that encode Rab family small GTPases that are involved in both the exocytic and endocytic recycling pathways. The cdc50-ts mutants did not exhibit major defects in the exocytic pathways, but they did exhibit those in endocytic recycling; large membranous structures containing the vesicle-soluble N-ethylmaleimide-sensitive factor attachment protein receptor Snc1p intracellularly accumulated in these mutants. Genetic results suggested that the YPT31/32 effector RCY1 and CDC50 function in the same signaling pathway, and simultaneous overexpression of CDC50, DRS2, and GFP-SNC1 restored growth as well as the plasma membrane localization of GFP-Snc1p in the rcy1Delta mutant. In addition, Rcy1p coimmunoprecipitated with Cdc50p-Drs2p. We propose that the Ypt31p/32p-Rcy1p pathway regulates putative phospholipid translocases to promote formation of vesicles destined for the trans-Golgi network from early endosomes.

Mutational analysis of the Lem3p-Dnf1p putative phospholipid-translocating P-type ATPase reveals novel regulatory roles for Lem3p and a carboxyl-terminal region of Dnf1p independent of the phospholipid-translocating activity of Dnf1p in yeast

Lem3p-Dnf1p is a putative aminophospholipid translocase (APLT) complex that is localized to the plasma membrane; Lem3p is required for Dnf1p localization to the plasma membrane. We have identified lem3 mutations, which did not affect formation or localization of the Lem3p-Dnf1p complex, but caused a synthetic growth defect with the null mutation of CDC50, a structurally and functionally redundant homologue of LEM3. Interestingly, these lem3 mutants exhibited nearly normal levels of NBD-labeled phospholipid internalization across the plasma membrane, suggesting that Lem3p may have other functions in addition to regulation of the putative APLT activity of Dnf1p at the plasma membrane. Similarly, deletion of the COOH-terminal cytoplasmic region of Dnf1p affected neither the localization nor the APLT activity of Dnf1p at the plasma membrane, but caused a growth defect in the cdc50Delta background. Our results suggest that the Lem3p-Dnf1p complex may play a role distinct from its plasma membrane APLT activity when it substitutes for the Cdc50p-Drs2p complex, its redundant partner in the endosomal/trans-Golgi network compartments.

Cdc50p, a protein required for polarized growth, associates with the Drs2p P-type ATPase implicated in phospholipid translocation in Saccharomyces cerevisiae

Cdc50p, a transmembrane protein localized to the late endosome, is required for polarized cell growth in yeast. Genetic studies suggest that CDC50 performs a function similar to DRS2, which encodes a P-type ATPase of the aminophospholipid translocase (APT) subfamily. At low temperatures, drs2Delta mutant cells exhibited depolarization of cortical actin patches and mislocalization of polarity regulators, such as Bni1p and Gic1p, in a manner similar to the cdc50Delta mutant. Both Cdc50p and Drs2p were localized to the trans-Golgi network and late endosome. Cdc50p was coimmunoprecipitated with Drs2p from membrane protein extracts. In cdc50Delta mutant cells, Drs2p resided on the endoplasmic reticulum (ER), whereas Cdc50p was found on the ER membrane in drs2Delta cells, suggesting that the association on the ER membrane is required for transport of the Cdc50p-Drs2p complex to the trans-Golgi network. Lem3/Ros3p, a homolog of Cdc50p, was coimmunoprecipitated with another APT, Dnf1p; Lem3p was required for exit of Dnf1p out of the ER. Both Cdc50p-Drs2p and Lem3p-Dnf1p were confined to the plasma membrane upon blockade of endocytosis, suggesting that these proteins cycle between the exocytic and endocytic pathways, likely performing redundant functions. Thus, phospholipid asymmetry plays an important role in the establishment of cell polarity; the Cdc50p/Lem3p family likely constitute potential subunits specific to unique P-type ATPases of the APT subfamily.

She4p/Dim1p interacts with the motor domain of unconventional myosins in the budding yeast, Saccharomyces cerevisiae

She4p/Dim1p, a member of the UNC-45/CRO1/She4p (UCS) domain-containing protein family, is required for endocytosis, polarization of actin cytoskeleton, and polarization of ASH1 mRNA in Saccharomyces cerevisiae. We show herein that She4p/Dim1p is involved in endocytosis and actin polarization through interactions with the type I myosins Myo3p and Myo5p. Two-hybrid and biochemical experiments showed that She4p/Dim1p interacts with the motor domain of Myo3/5p through its UCS domain. She4p/Dim1p was required for Myo5p localization to cortical patch-like structures. Using random mutagenesis of the motor region of MYO5, we identified four independent dominant point mutations that suppress the temperature-sensitive growth phenotype of the she4/dim1 null mutant. All of the amino acid substitutions caused by these mutations, V164I, N168I, N209S, and K377M, could suppress the defects of endocytosis and actin polarization of the she4/dim1 mutant as well. She4p/Dim1p also showed two-hybrid interactions with the motor domain of a type II myosin Myo1p and type V myosins Myo2p and Myo4p, and was required for proper localization of Myo4p, which regulates polarization of ASH1 mRNA. Our results suggest that She4p/Dim1p is required for structural integrity or regulation of the motor domain of unconventional myosins.

The upstream regulator, Rsr1p, and downstream effectors, Gic1p and Gic2p, of the Cdc42p small GTPase coordinately regulate initiation of budding in Saccharomyces cerevisiae

BACKGROUND

Cdc42p, a Rho family small GTPase, is essential for budding initiation in the yeast Saccharomyces cerevisiae. The homologous proteins Gic1p and Gic2p (Gic1/2p) are effectors of Cdc42p, but their precise functions remain unknown. Rsr1p/Bud1p is a Ras family small GTPase that controls the selection of the budding site. Previous observations suggested that Rsr1p-GTP recruits Cdc24p, a GDP/GTP exchange factor for Cdc42p, at the incipient bud site. However, this model only addresses how Rsr1p determines the budding site, because the rsr1 mutant normally initiates budding.

RESULTS

Here we show that a rsr1 gic1 gic2 mutant fails to initiate budding, resulting in unbudded, large, and multinucleated cells. Expression of a dominant active or dominant negative mutant of RSR1 also inhibited the growth of the gic1 gic2 mutant, suggesting that cycling of Rsr1p between the GTP- and GDP-bound forms is required for budding initiation in the gic1 gic2 mutant. Among the mutations in effectors of CDC42, only the gic1 gic2 mutation demonstrated a synthetic lethal interaction with rsr1. Increased gene dosage of CDC42 suppressed defects in budding initiation of rsr1 gic1 gic2 mutants containing additional mutations in other effectors of CDC42, including BNI1, CLA4 or STE20. The polarized localization of Bni1p-GFP (green fluorescent protein) and Cla4p-GFP was lost after depletion of Gic1p in the rsr1 gic2 mutant.

CONCLUSION

We propose that Gic1/2p may stabilize or maintain a complex consisting of Cdc42p-GTP and its effectors at the budding site, which are assembled by the action of the Rsr1p-Cdc24p system.

Cdc50p, a conserved endosomal membrane protein, controls polarized growth in Saccharomyces cerevisiae

During the cell cycle of the yeast Saccharomyces cerevisiae, the actin cytoskeleton and the growth of cell surface are polarized, mediating bud emergence, bud growth, and cytokinesis. We identified CDC50 as a multicopy suppressor of the myo3 myo5-360 temperature-sensitive mutant, which is defective in organization of cortical actin patches. The cdc50 null mutant showed cold-sensitive cell cycle arrest with a small bud as reported previously. Cortical actin patches and Myo5p, which are normally localized to polarization sites, were depolarized in the cdc50 mutant. Furthermore, actin cables disappeared, and Bni1p and Gic1p, effectors of the Cdc42p small GTPase, were mislocalized in the cdc50 mutant. As predicted by its amino acid sequence, Cdc50p appears to be a transmembrane protein because it was solubilized from the membranes by detergent treatment. Cdc50p colocalized with Vps21p in endosomal compartments and was also localized to the class E compartment in the vps27 mutant. The cdc50 mutant showed defects in a late stage of endocytosis but not in the internalization step. It showed, however, only modest defects in vacuolar protein sorting. Our results indicate that Cdc50p is a novel endosomal protein that regulates polarized cell growth.

The novel adaptor protein, Mti1p, and Vrp1p, a homolog of Wiskott-Aldrich syndrome protein-interacting protein (WIP), may antagonistically regulate type I myosins in Saccharomyces cerevisiae

Type I myosins in yeast, Myo3p and Myo5p (Myo3/5p), are involved in the reorganization of the actin cytoskeleton. The SH3 domain of Myo5p regulates the polymerization of actin through interactions with both Las17p, a homolog of mammalian Wiskott-Aldrich syndrome protein (WASP), and Vrp1p, a homolog of WASP-interacting protein (WIP). Vrp1p is required for both the localization of Myo5p to cortical patch-like structures and the ATP-independent interaction between the Myo5p tail region and actin filaments. We have identified and characterized a new adaptor protein, Mti1p (Myosin tail region-interacting protein), which interacts with the SH3 domains of Myo3/5p. Mti1p co-immunoprecipitated with Myo5p and Mti1p-GFP co-localized with cortical actin patches. A null mutation of MTI1 exhibited synthetic lethal phenotypes with mutations in SAC6 and SLA2, which encode actin-bundling and cortical actin-binding proteins, respectively. Although the mti1 null mutation alone did not display any obvious phenotype, it suppressed vrp1 mutation phenotypes, including temperature-sensitive growth, abnormally large cell morphology, defects in endocytosis and salt-sensitive growth. These results suggest that Mti1p and Vrp1p antagonistically regulate type I myosin functions.

A Drosophila MAPKKK, D-MEKK1, mediates stress responses through activation of p38 MAPK

In cultured mammalian cells, the p38 mitogen-activated protein kinase (MAPK) pathway is activated in response to a variety of environmental stresses. How ever, there is little evidence from in vivo studies to demonstrate a role for this pathway in the stress response. We identified a Drosophila MAPK kinase kinase (MAPKKK), D-MEKK1, which can activate p38 MAPK. D-MEKK1 is structurally similar to the mammalian MEKK4/MTK1 MAPKKK. D-MEKK1 kinase activity was activated in animals under conditions of high osmolarity. Drosophila mutants lacking D-MEKK1 were hypersensitive to environmental stresses, including elevated temperature and increased osmolarity. In these D-MEKK1 mutants, activation of Drosophila p38 MAPK in response to stress was poor compared with activation in wild-type animals. These results suggest that D-MEKK1 regulation of the p38 MAPK pathway is critical for the response to environmental stresses in Drosophila.

Distortion of proximodistal information causes JNK-dependent apoptosis in Drosophila wing

Distinct and evolutionarily conserved signal-transduction cascades mediate the survival or death of cells during development. The c-Jun amino-terminal kinases (JNKs) of the mitogen-activated protein kinase superfamily are involved in apoptotic signalling in various cultured cells. However, the role of the JNK pathway in development is less well understood. In Drosophila, Decapentaplegic (Dpp; a homologue of transforming growth factor-beta) and Wingless (Wg; a Wnt homologue) proteins are secretory morphogens that act cooperatively to induce formation of the proximodistal axis of appendages. Here we show that either decreased Dpp signalling in the distal wing cells or increased Dpp signalling in the proximal wing cells causes apoptosis. Inappropriate levels of Dpp signalling lead to aberrant morphogenesis in the respective wing zones, and these apoptotic zones are also determined by the strength of the Wg signal. Our results indicate that distortion of the positional information determined by Dpp and Wg signalling gradients leads to activation of the JNK apoptotic pathway, and the consequent induction of cell death thereby maintains normal morphogenesis.

Endoplasmic reticulum membrane localization of Rce1p and Ste24p, yeast proteases involved in carboxyl-terminal CAAX protein processing and amino-terminal a-factor cleavage

Proteins terminating in the CAAX motif, for example Ras and the yeast a-factor mating pheromone, are prenylated, trimmed of their last three amino acids, and carboxyl-methylated. The enzymes that mediate these activities, collectively referred to as CAAX processing components, have been identified genetically in Saccharomyces cerevisiae. Whereas the Ram1p/Ram2p prenyltransferase is a cytosolic soluble enzyme, sequence analysis predicts that the other CAAX processing components, the Rce1p and Ste24p proteases and the Ste14p methyltransferase, contain multiple membrane spans. To determine the intracellular site(s) at which CAAX processing occurs, we have examined the localization of the CAAX proteases Rce1p and Ste24p by subcellular fractionation and indirect immunofluorescence. We find that both of these proteases are associated with the endoplasmic reticulum (ER) membrane. In addition to having a role in CAAX processing, the Ste24p protease catalyzes the first of two cleavage steps that remove the amino-terminal extension from the a-factor precursor, suggesting that the first amino-terminal processing step of a-factor maturation also occurs at the ER membrane. The ER localization of Ste24p is consistent with the presence of a carboxyl-terminal dilysine ER retrieval motif, although we find that mutation of this motif does not result in mislocalization of Ste24p. Because the ER is not the ultimate destination for a-factor or most CAAX proteins, our results imply that a mechanism must exist for the intracellular routing of CAAX proteins from the ER membrane to other cellular sites.

Dual roles for Ste24p in yeast a-factor maturation: NH2-terminal proteolysis and COOH-terminal CAAX processing

Maturation of the Saccharomyces cerevisiae a-factor precursor involves COOH-terminal CAAX processing (prenylation, AAX tripeptide proteolysis, and carboxyl methylation) followed by cleavage of an NH2-terminal extension (two sequential proteolytic processing steps). The aim of this study is to clarify the precise role of Ste24p, a membrane-spanning zinc metalloprotease, in the proteolytic processing of the a-factor precursor. We demonstrated previously that Ste24p is necessary for the first NH2-terminal processing step by analysis of radiolabeled a-factor intermediates in vivo (Fujimura-Kamada, K., F.J. Nouvet, and S. Michaelis. 1997. J. Cell Biol. 136:271-285). In contrast, using an in vitro protease assay, others showed that Ste24p (Afc1p) and another gene product, Rce1p, share partial overlapping function as COOH-terminal CAAX proteases (Boyartchuk, V.L., M.N. Ashby, and J. Rine. 1997. Science. 275:1796-1800). Here we resolve these apparently conflicting results and provide compelling in vivo evidence that Ste24p indeed functions at two steps of a-factor maturation using two methods. First, direct analysis of a-factor biosynthetic intermediates in the double mutant (ste24Delta rce1Delta) reveals a previously undetected species (P0*) that fails to be COOH terminally processed, consistent with redundant roles for Ste24p and Rce1p in COOH-terminal CAAX processing. Whereas a-factor maturation appears relatively normal in the rce1Delta single mutant, the ste24Delta single mutant accumulates an intermediate that is correctly COOH terminally processed but is defective in cleavage of the NH2-terminal extension, demonstrating that Ste24p is also involved in NH2-terminal processing. Together, these data indicate dual roles for Ste24p and a single role for Rce1p in a-factor processing. Second, by using a novel set of ubiquitin-a-factor fusions to separate the NH2- and COOH-terminal processing events of a-factor maturation, we provide independent evidence for the dual roles of Ste24p. We also report here the isolation of the human (Hs) Ste24p homologue, representing the first human CAAX protease to be cloned. We show that Hs Ste24p complements the mating defect of the yeast double mutant (ste24Delta rce1Delta) strain, implying that like yeast Ste24p, Hs Ste24p can mediate multiple types of proteolytic events.

A novel membrane-associated metalloprotease, Ste24p, is required for the first step of NH2-terminal processing of the yeast a-factor precursor

Many secreted bioactive signaling molecules, including the yeast mating pheromones a-factor and alpha-factor, are initially synthesized as precursors requiring multiple intracellular processing enzymes to generate their mature forms. To identify new gene products involved in the biogenesis of a-factor in Saccharomyces cerevisiae, we carried out a screen for MA Ta-specific, mating-defective mutants. We have identified a new mutant, ste24, in addition to previously known sterile mutants. During its biogenesis in a wild-type strain, the a-factor precursor undergoes a series of COOH-terminal CAAX modifications, two sequential NH2-terminal cleavage events, and export from the cell. Identification of the a-factor biosynthetic intermediate that accumulates in the ste24 mutant revealed that STE24 is required for the first NH2-terminal proteolytic processing event within the a-factor precursor, which takes place after COOH-terminal CAAX modification is complete. The STE24 gene product contains multiple predicted membrane spans, a zinc metalloprotease motif (HEXXH), and a COOH-terminal ER retrieval signal (KKXX). The HEXXH protease motif is critical for STE24 activity, since STE24 fails to function when conserved residues within this motif are mutated. The identification of Ste24p homologues in a diverse group of organisms, including Escherichia coli, Schizosaccharomyces pombe, Haemophilus influenzae, and Homo sapiens, indicates that Ste24p has been highly conserved throughout evolution. Ste24p and the proteins related to it define a new subfamily of proteins that are likely to function as intracellular, membrane-associated zinc metalloproteases.