(Received for publication, August 22, 1995; and in revised form, October 4, 1995)
From the
In Aspergillus nidulans, the regulation of gene expression in response to changes in ambient pH is mediated by the PacC zinc finger transcriptional regulator. At alkaline ambient pH, PacC is proteolytically processed to a functional form serving as an activator of alkaline-expressed genes and a repressor of acid-expressed genes. This activation of PacC occurs in response to a signal mediated by the products of the pal genes. Thus, the products of the palA, -B, -C, -F, -H, and -I genes constitute an alkaline ambient pH signal transduction pathway. How the pal signal transduction pathway senses ambient pH and transduces a signal to trigger PacC processing is a fascinating unresolved problem. We have cloned and sequenced the palB gene. The predicted palB gene product has similarity to the catalytic domain of the calpain family of calcium-activated cysteine proteases. We have shown, however, that the PalB protein does not catalyze the final step of proteolytic processing of PacC.
The ascomycete Aspergillus nidulans, like many other microorganisms, grows over a wide pH range(1, 2) . Crucial to this physiological versatility, A. nidulans has a regulatory system to ensure that gene products that function beyond cell boundaries, such as permeases and extracellular enzymes, are only synthesized at pHs at which they can function effectively so that, for example, acid phosphatase is secreted in acidic environments and alkaline phosphatase in alkaline environments(1, 3, 4) . Mutations in the pacC and palA, -B, -C, -F, -H, and -I genes alter the response to ambient pH. The PacC zinc finger transcriptional regulator directly regulates expression of genes under pH control, and the products of the pal genes constitute a signal transduction pathway which, at alkaline ambient pH, triggers the conversion of PacC to a functional form as a transcriptional activator of alkaline-expressed genes and repressor of acid-expressed genes(1, 3, 4, 5) . The absence of the pal signal at acid pH results in derepression of acid-expressed genes and lack of activation of alkaline-expressed genes. We are characterizing the genes of this novel ambient pH signal transduction pathway. We have cloned the palB gene and shown that the predicted PalB protein has sequence similarity to the catalytic domain of the calpain family of calcium-activated cysteine proteases. Thus, a cysteine protease is a component of the alkaline ambient pH signal transduction pathway in Aspergillus.
Figure 1:
Partial restriction map of palB genomic region, fragments that transform palB7 strains to pal and palB gene disruption. A, restriction map and transforming fragments. S, SphI; X, XbaI; R, EcoRI; B, BamHI; K, KpnI. The striped
bar indicates the XbaI/KpnI fragment used to
probe Northern blots and screen for palB cDNAs; the EcoRI/BamHI fragment contained within it was also
used as a probe for Northern blots and in cDNA screening. The BamHI and SphI fragments above the map both transform palB7 strains to pal
, but the SphI fragment does so at a higher frequency. B, palB gene disruption. The arrow indicates the
direction and location of the palB transcriptional unit. For
disruption with a linear restriction fragment, a pyr4-containing BglII fragment was cloned into the BamHI site (see ``Materials and Methods''). Strains
tested for growth on minimal medium pH 6.5 (left) and pH 8 (right) are: 1, pabaA1 argB2 areA49 chaA1 palB7; 2, pabaA1 argB2 areA49 chaA1 palB7::pILJ16.1 (palB7 mutant strain transformed with
plasmid pILJ16.1, which contains the 6-kb BamHI restriction
fragment shown above); 3, yA2 pabaA1 pyrG89::pUC19.18
BamHI (palB gene
disruption); 4, yA2 pabaA1 pyrG89
(recipient for palB gene disruption); 5, diploid yA2 pabaA1 pyrG89::pUC19.18
BamHI/argB2 inoB2 chaA1 palB7; and 6, diploid yA2 pabaA1 pyrG89::pUC19.18
BamHI/biA1 ahrA2 pantoA10.
We cloned the palB gene using linkage group VIII
clones from a chromosome-allocated, wild type A. nidulans cosmid library(8) . To identify palB transformants, we used the inability of pal mutants to grow on alkaline pH media(1) . The
cosmid clone pWE15 03H12 rescued the palB7 strain to give a pal
phenotype. This cosmid clone was shown by
hybridization to overlap cosmid clones containing the ivoB gene, which is closely linked (2.1 centimorgans(11) ) to palB (data not shown). A 6-kb BamHI restriction
fragment of the cosmid was then found to rescue the palB7
strain (Fig. 1A). When this BamHI fragment was
inserted into the argB
-containing plasmid
pILJ16 and the resulting plasmid (pILJ16.1) used for transformation,
all pal
transformants analyzed had BamHI fragment-directed homologous integration (data not
shown). One transformant, showing homologous integration, was crossed
to an argB2 strain. All 104 progeny analyzed were palB
, and the argB
and chaA1 markers showed 8% recombination, consistent
with the linkage (12) between palB and chaA.
This and the result that the larger, overlapping SphI
restriction fragment rescued at a higher frequency suggested that the
entire palB gene might not be contained within the BamHI fragment and that the BamHI site within the SphI fragment might be within the palB gene (Fig. 1). This BamHI site was therefore used for gene
disruption (Fig. 1B). Strains disrupted at the BamHI site are unable to grow on pH 8 medium (Fig. 1B). Also, the recessive disruption allele is
non-complementing in a diploid with palB7 (Fig. 1B). When a disruptant was crossed to a tubA1 pantoA10 strain, analysis of 104 progeny
located the palB disruption mutation between tubA and pantoA, consistent with the map position (6) of palB. These results confirmed that palB had been
cloned and that the BamHI site is within the gene.
The sequence for 3.2 kb of the palB-containing genomic region is shown in Fig. 2. This sequence spans the BamHI site within palB. The 3-kb XbaI/KpnI restriction fragment indicated by the striped bar in Fig. 1and the 700-base pair EcoRI/BamHI restriction fragment contained within it were used to probe Northern blots of wild type A. nidulans and to screen a cDNA library. Both probes hybridized to a message of approximately 3 kb. The boundaries of the largest cDNA clone isolated are shown by arrows in Fig. 2. We believe this cDNA clone contains all of the palB coding region for the following reasons. The clone contains a poly(A) tail, and all three reading frames upstream of the putative start codon are closed multiple times in the cDNA. Also, the size of this cDNA clone is close to our estimate of the mRNA size. This cDNA clone, a shorter cDNA clone, and reverse transcriptase PCR fragments were sequenced to determine the position of introns (Fig. 2). The resulting open reading frame would encode a protein of 842 amino acids and a molecular mass of 93.9 kDa. Sequence data base searches with this open reading frame using the FASTA program (13) retrieve members of the calpain family of calcium-activated cysteine proteases. Strong sequence similarity between PalB and the calpains is limited to the catalytic region. A multiple sequence alignment of the catalytic region of several calpains and PalB is shown in Fig. 3. In addition to the Cys, His, and Asn residues believed to constitute the calpain catalytic triad(14) , starred in Fig. 3, PalB contains many conserved residues within the catalytic domain. In a pairwise comparison with human calpain p94, PalB has 29.1% amino acid identity over 300 amino acids and 32.3% identity over 189 amino acids of the catalytic domain. As indicated in Fig. 3, the BamHI site used for disruption separates the codon for the Cys catalytic residue from those for the His and Asn catalytic residues.
Figure 2: Nucleotide and derived amino acid sequence of palB. The XbaI, EcoRI, BamHI, and KpnI restriction sites are those shown in Fig. 1. The putative catalytic residues are circled. The boundaries of the largest cDNA clone isolated are indicated by arrows. The 5` cDNA boundary corresponds to the beginning of the sequence. The 3` arrow indicates the site of polyadenylation.
Figure 3: Alignment of partial PalB sequence with calpain catalytic domains. Amino acid residues identical or conserved (D-E, Q-N, A-G, R-K, L-M-I-V, F-Y-W, S-T) between PalB and at least one other calpain are boxed. The three catalytic residues are indicated by stars. A putative EF-hand (17) is overlined. The BamHI site used for gene disruption is indicated. The numbers refer to PalB residues. Calpains shown are: Cap1, human calpain (P07384); nCL-2, rat calpain nCL2 (B14479); p94, human calpain p94 (P20807); Dros, D. melanogaster CalpA (Z46891 and Z46892); Schis, Schistosoma mansoni calpain (P27730); Sol, D. melanogaster Sol protein (M64084). Accession numbers are given in parentheses. Sequence alignment was done with the GCG Pileup program(26) .
Activation of PacC in response to alkaline pH requires PacC
proteolysis (4) . A 29-kDa form of PacC, which is
functional as a transcriptional activator and repressor, predominates
at alkaline pH in wild type strains and irrespective of pH in pacC
mutant strains, which mimic the effects of
growth at alkaline pH. This 29-kDa form of PacC migrates as complex I
in gel mobility shift experiments using a promoter fragment of the
PacC-regulated ipnA (isopenicillin N synthetase) gene and
extracts of a wild type strain grown under alkaline conditions (Fig. 4, lane 3) or an alkalinity-mimicking pacC
202 strain grown under acidic conditions (Fig. 4, lane 4). The full-length (73 kDa) form of
PacC, which is inactive as a transcriptional activator and repressor,
migrates more slowly as complex II and predominates in gel mobility
shift assays with extracts of wild type strains grown under acidic
conditions (4) or palB7 strains grown under acidic or
neutral conditions (Fig. 4, lanes 5 and 6).
Additionally, pal mutants, including palB7 strains,
produce greatly reduced amounts of PacC DNA binding activity (Fig. 4, lanes 5 and 6 and (4) ). We
have shown previously that pacC
mutations are
phenotypically epistatic to mutations in the pal signal
transduction pathway genes(1, 3, 5) , and the
expectation would therefore be that pacC
epistasis
would extend to PacC processing. This prediction is testable because
proteolytic processing of PacC to yield the 29-kDa form occurs in
alkalinity-mimicking pacC
strains in which the
primary translation product is already truncated to some extent by pacC
mutations(4) . The prediction has
been previously confirmed in the case of a mutation in palA(4) . Despite the phenotypic epistasis of pacC
mutations to palB mutations(1, 3) , it was necessary to determine
experimentally whether processing of PacC to the 29-kDa form is
affected by a palB mutation because PalB appears to be a
cysteine protease. PalB is clearly not responsible for this processing,
as the fully proteolyzed form of PacC predominates at acid pH in gel
mobility shifts using extracts of a strain carrying both pacC
202 and palB7 mutations (Fig. 4, lanes 7 and 8).
Figure 4:
PalB is not responsible for the final
proteolysis converting PacC to the functional form. Electrophoretic
mobility shift assays were performed using a 31-base pair promoter
fragment of the PacC-regulated ipnA (isopenicillin A
synthetase) gene containing the high affinity ipnA2 PacC-binding site
and extracts of various strains as described(4) . Lane
1, oligonucleotide alone; lanes 2 and 3, 5
µg of protein of wild type (biA1) grown in acidic and
alkaline conditions, respectively; lane 4, 5 µg of protein
of the pabaA1 pacC202 strain grown in
acidic conditions; lanes 5 and 6, 10 µg of
protein of the pabaA1 palB7 strain grown in acidic
and neutral conditions, respectively (palB7 strains do not
grow in alkaline conditions); lanes 7 and 8, 2.5 and
5 µg of protein, respectively, of the strain inoB2 pacC
202 chaA1 palB7 grown in
acidic conditions. Solid arrows indicate the high mobility
complex I, formed with the functional 29-kDa proteolyzed version of
PacC, and the low mobility complex II, formed with the full-length,
non-functional version of PacC. Because pacC
202
results in a substantially shorter full-length version of PacC it
increases the mobility of complex II(3, 4) . The open arrow indicates a complex probably formed by a
proteolysis degradation product of the 29-kDa functional version of
PacC. As seen from lanes 7 and 8, the level of this
putative degradation product is not diminished (and might even be
elevated) by the palB7 mutation. The slight amount of complex
I formed by neutral grown palB7 extracts (lane 6 and
Fig. 7 of (4) ) also forms when extracts of a neutral grown palB disruption strain are used (data not shown), indicating
that very limited PacC processing can occur in the total absence of
PalB. Thus, although the sequence change resulting from the palB7 mutation has yet to be determined, its physiological and
biochemical phenotype appears to be that of a null
allele.
We have shown that a protein with sequence similarity to calpains is a component of the alkaline ambient pH signal transduction pathway in Aspergillus. Calpains contain C-terminal EF-hand structures which are responsible for calcium binding and regulation. PalB, like the Drosophila melanogaster Sol (small optic lobes protein) has similarity to the catalytic domain of calpains but not to these calcium-binding domains(15) . However, PalB and Sol both have similarity to a putative EF-hand structure located immediately C-terminal to the catalytic domain (overlined in Fig. 3). Some putative EF-hand structures in known calpains deviate from the EF-hand consensus(16, 17) . Allowing for such deviations, there are also several possible EF-hands in the C-terminal region of PalB.
As hypothesized for the Sol protein(15) , PalB might have substrate specificity similar to that of calpains but not be calcium-regulated. In vivo and in vitro substrates for calpains include cytoskeletal proteins, transcription factors, protein kinases, and protein phosphatases(18, 19, 20, 21, 22, 23) . Protein kinase C is proteolyzed to a permanently active form by calpain(19) . Certain of these substrates are suggestive of signal transduction roles for calpains. Also suggestive of a calpain signal transduction role is the nuclear location of a muscle-specific calpain, which is mutated in one form of muscular dystrophy(24, 25) .
PalB is required for growth at
alkaline but not acidic pH. Although the palB mRNA is in low
abundance, we have determined that there is little or no difference in
its levels after growth in the pH range 4-8 (data not shown). The
final proteolytic processing step for PacC is not catalyzed by PalB.
The pal pathway is, however, responsible for a PacC
modification that is required for its conversion to a functional form
at alkaline pH(3, 4) . The alkalinity-mimicking pacC202 mutation removes 214 C-terminal residues
from PacC(3) . This deletion allows proteolytic processing of
PacC to the 29-kDa functional form in the absence of the pal signal at acidic pH or in a pal mutant background ( Fig. 4and (4) ). The pacC
202 and
other pacC
mutation C-terminal truncations might
destabilize intramolecular interactions, allowing proteolysis of PacC
to the active form(4) . In wild type strains, the pal pathway is thought to introduce a modification of PacC at alkaline
pH, disrupting intramolecular interactions to allow activating
proteolysis (4) . The type of PacC modification mediated by the pal pathway in response to alkaline pH is not known, but an
earlier (and more C-terminal) proteolytic cleavage of PacC resulting in
susceptibility to further proteolysis might be mediated by PalB.
Alternatively, PalB might proteolyze one of the other pal gene
products in a signaling cascade. Sequencing of other pal genes
and biochemical studies of PalB might help identify the PalB
substrate(s).
The nucleotide sequence(s) reported in this paper has been submitted to the GenBank(TM)/EMBL Data Bank with accession number(s) Z54244[GenBank].