(Received for publication, January 24, 1997, and in revised form, February 25, 1997)
From the Banting and Best Department of Medical
Research, University of Toronto, Charles H. Best Institute, Toronto,
Ontario M5G1L6, Canada and the ¶ Department of Medicine and
Pathophysiology, Osaka University Medical School, 2-2 Yamadaoka,
Suita, Osaka 565, Japan
Phospholamban (PLN), a homopentameric, integral membrane protein, reversibly inhibits cardiac sarcoplasmic reticulum Ca2+-ATPase (SERCA2a) activity through intramembrane interactions. Here, alanine-scanning mutagenesis of the PLN transmembrane sequence was used to identify two functional domains on opposite faces of the transmembrane helix. Mutations in one face diminish inhibitory interactions with transmembrane sequences of SERCA2a, but have relatively little effect on the pentameric state, while mutations in the other face activate inhibitory interactions and enhance monomer formation. Double mutants are monomeric, but loss of inhibitory function is dominant over activation of inhibitory function. These observations support the proposal that the SERCA2a interaction site lies on the helical face which is not involved in pentamer formation. Four highly inhibitory mutants are effectively devoid of pentamer, suggesting that pentameric PLN represents a less active or inactive reservoir that dissociates to provide inhibitory monomeric PLN subunits. A model is presented in which the degree of PLN inhibition of SERCA2a activity is ultimately determined by the concentration of the inhibited PLN monomer·SERCA2a heterodimeric complex. The concentration of this inhibited complex is determined by the dissociation constant for the PLN pentamer (which is mutation-sensitive) and by the dissociation constant for the PLN/SERCA2a heterodimer (which is likely to be mutation-sensitive).
Phospholamban (PLN)1 is a 52-amino acid, integral membrane protein (1) that interacts with and reversibly inhibits the activity of the cardiac sarcoplasmic reticulum Ca2+-ATPase (SERCA2a). In this role, it is a major regulator of the kinetics of cardiac contractility (2). PLN has the mobility of a homopentamer in SDS gels, but the pentamer is dissociated to monomers by boiling in SDS (3). It is an open question whether the functional inhibitory unit is a pentamer or a monomer and whether pentamers and monomers are in dynamic equilibrium in the sarcoplasmic reticulum membrane.
Much attention has been directed toward the phosphorylation sites on Ser16 and Thr17 in cytoplasmic domain Ia of PLN and their role in regulating the inhibitory function of PLN (4, 5). In earlier studies we showed that the PLN cytoplasmic interaction site is formed by charged and hydrophobic amino acids 1-20 (6), while the complementary SERCA2a interaction site consists of amino acids Lys-Asp-Asp-Lys-Pro-Val402 (7).
In a later evaluation of potential transmembrane interaction sites (8), we coexpressed SERCA2a with PLN transmembrane sequences 31-52 (PLN domain II) or with PLN domain II constructs to which NH2-terminal, cytoplasmic epitopes such as PLN1-20 or hemagglutinin were fused. We found that the inhibitory interaction site lies entirely in the transmembrane sequences of PLN and SERCA2a, but can be modulated, through long range interactions, by the noninhibitory cytoplasmic interaction site. We also discovered the phenomenon of "supershifting," in which the apparent affinity of specific PLN mutants for SERCA2a is enhanced, so that Ca2+ concentrations well above the physiological range are required to reverse the inhibitory action of the mutant PLN. In the present study, we associate gain of function with PLN mutants that promote PLN depolymerization. Since the most inhibitory mutants of PLN are devoid of pentamer, we deduce that the pentamer represents a less active or inactive reservoir of subunits and that the PLN monomer is the functional, inhibitory form.
For mutagenesis, a
172-base pair fragment containing the coding sequence for rabbit PLN
(bases 6 to 162 (9) was amplified in a recombinant polymerase chain
reaction (10) using primers with 5
-add-on sequences containing
restriction endonuclease sites for XbaI (5
end) and
SalI (3
end). The product was subcloned into pBluescript
KS+ (Stratagene) after digestion with XbaI and SalI. Mutagenesis with this construct as a template was
carried out as described previously (11, 12). Wild type and mutant PLN
cDNAs were ligated into the XbaI and SalI
sites of the pMT2 expression vector (13) and amplified. Plasmid DNA was
purified by adsorption to and elution from Qiagen tip-500 columns. PLN and SERCA2a cDNAs in the pMT2 vector were cotransfected (8 µg of
each cDNA per dish) into HEK-293 cells using the calcium phosphate precipitation method (14). Cells were harvested 48 h after
transfection, and microsomes were prepared and assayed for
Ca2+ transport activity as described previously (6). Data
were analyzed as described previously (8).
Immunoblotting of SERCA2a and wild type and mutant forms of PLN expressed in HEK-293 cell microsomes was carried out with 10 µg of microsomal proteins. Proteins were solubilized in SDS buffer at room temperature, separated on 12.5% SDS-PAGE, transferred electrophoretically to nitrocellulose membranes, and incubated with monoclonal antibody IID8F6 against SERCA2a or antibody 1D11 against PLN (15), as described previously (8), except that PROTRANTM nitrocellulose membranes with a pore size of 0.05 microns (Schleicher & Schuell) were used during protein transfer. Antibody binding was detected by chemiluminescence using an ECL Western blotting detection system (Amersham Corp). Oligomer/monomer ratios were calculated by scanning densitometry of individual lanes in the exposed films of the immunoblots of the gels.
In earlier studies (8) we demonstrated that inhibitory
interactions between PLN and SERCA2a occur through intramembrane interactions. To characterize these inhibitory interactions, we mutated
each of PLN transmembrane (domain II) amino acids, Leu31
through Leu52, to Ala and expressed the mutant PLN
cDNAs with SERCA2a cDNA in HEK-293 cells. Microsomes were
isolated from transfected cells and Ca2+ dependence of
Ca2+ transport was measured for each isolate. The reduction
in the apparent affinity of SERCA2a for Ca2+
(KCa expressed in
pCa units) provided a
measure of the inhibitory function of wild type or mutated PLN (Fig.
1A). The
KCa for
PLN was
0.33 pCa units. PLN mutants, L31A, N34A, F35A, I38A, L42A, I48A, V49A, and L52A, had diminished inhibitory function relative to
PLN, as indicated by
KCa values between 0 and
about
0.2 pCa units. PLN mutants, F32A, I33A, L37A, I40A, L43A, L44A,
I47A, M50A, and L51A, gained inhibitory function relative to PLN, as indicated by
KCa values between about
0.43
and
0.8 pCa units. PLN mutants, C36A, L39A, C41A, I45A and C46A, had
unaltered function, as indicated by
KCa
values not significantly different from
0.33 pCa units. Loss or gain
of PLN function could not be correlated with different levels of
expression (Fig. 1B). Moreover, supershifting was not
induced by overexpression of PLN (8).
We have assumed that residues Leu31 to Leu52
form an -helix (16-19), and in Fig. 1A, we have numbered
them in four linear sequences (1-7, 1-7, 1-7, and
1) corresponding to repeating positions in an
-helix with
3.5 residues per turn. Loss and gain of function of the PLN mutants is
seen to be cyclical and progressive, repeating every three or four
residues, as would be expected if loss of function were associated with
mutations on one face of the PLN domain II helix and gain of function
were associated with mutations on the opposite face.
Wild type PLN is about 75% pentameric in SDS-PAGE (Fig. 1B, Table I), but is dissociated to a monomer by boiling in SDS under reducing or nonreducing conditions (3). Mutants of Leu37, Ile40, Leu44, and Ile47 are monomeric without boiling, as suggested by their mobility in SDS-PAGE (16, 19). These studies have led to structural models in which these residues lie on one face of the PLN domain II helix and play a role in pentamer formation (18, 19).
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In Fig. 1B, we show the results of SDS-PAGE analysis of the pentamer stability of unboiled samples of PLN and each of the 22 PLN mutants. Estimates of the percent monomer (Table I) show that the mutants can be divided into four broad classes. Class 1 has the same or less monomer than wild type PLN; Class 2 has roughly twice as much monomer as PLN; Class 3 has roughly three times as much monomer as PLN; and Class 4 has roughly four times as much monomer as PLN, being virtually 100% monomer. As in Fig. 1A, we have numbered the PLN mutants in Fig. 1B in four linear sequences of seven residues. Loss and conservation of pentamer stability, like loss and gain of function, is cyclical and progressive, repeating every three or four residues, as would be expected if loss of stability were associated with mutations on one face of the PLN domain II helix. The cyclical changes of loss and gain of function and of pentamer stability are clearly in phase.
When the location of each mutated amino acid is plotted on a helical
wheel representing the PLN domain II helix (Fig. 2), it
is apparent that different classes of mutants are clustered. Loss of
function mutants lie on one face of the helix (the proposed exterior
face of each helix in a PLN pentamer (18, 19)). Measurement of percent
monomer places all of these residues in Classes 1 or 2. Gain of
function mutants all lie on the opposite face of the helix (the
proposed interior face of each helix in a PLN pentamer (18, 19)).
Measurement of percent monomer places all of these mutants in Classes 3 or 4. Measurement of percent monomer in the five mutations that did not
alter function placed them in Classes 1, 2, and 3.
We propose that the loss of function that occurs with each of the eight mutants, L31A, N34A, F35A, I38A, L42A, I48A, V49A, and L52A, on the exterior face of the helix, reflects the fact that each of these amino acids is essential for the interaction of the PLN domain II helix with complementary amino acids in the hydrophobic, transmembrane helices in SERCA2a. Ile45 and Cys41 lie on this helical face, but their function was unaltered by mutation. Ile45 (forming a Class 1 mutant) may have no suitable complement in the helical transmembrane sequences of SERCA2a and may play no role in interhelix interactions. The C41A mutant is substantially depolymerized (Class 3), but function is unaltered. This suggests that depolymerization is not the only requirement for activation of inhibitory function. The next steps must be formation of a PLN monomer·SERCA2a complex, followed by the formation of inhibitory interactions between specific amino acids. The ultimate stability of the inhibited PLN monomer·SERCA2a complex will also determine the degree of gain of inhibitory function.
Mutation of Cys36, Leu39, or Cys46 did not alter inhibitory function, but doubled the extent of depolymerization, placing these mutants in Class 2. These residues lie on an ill-defined boundary between the two functional domains where, in these cases, mutation does not seem to be particularly critical for either inhibitory function or pentamer formation. The definition of this boundary is of interest, because the Class 3, gain of function mutant, F32A, is clearly not a nonfunctional resident of the "boundary" area. Perhaps there is a distortion of the transmembrane helix near its cytoplasmic boundary, which aligns Phe32 with the interior face of the rest of the helix or the bulky side chain of Phe32 might, itself, distort the boundary.
All three Cys to Ala mutations destabilized the pentamer (Fig. 1B and Ref. 20), but retained normal function (Fig. 1A). Since Cys is "tiny", whereas residues at each of the other 19 positions are "small" (Val, Asn) or "bulky" (Leu, Ile, Met, Phe) (21), the mutation of Cys to Ala would not alter the bulk of the side chain significantly. This may account, in part, for their unaltered function. However, we do not know enough about the full effects of these mutant proteins on the formation of an inhibited PLN monomer·SERCa2a complex to understand their full meaning, nor do we understand why mutation of all three of these residues destabilizes the pentamer so effectively, when they are not believed to be key stabilizing residues in the PLN pentamer (18, 19).
The nine Class 3 and 4 mutants, I33A, F32A, L37A, I40A, L43A, L44A, I47A, M50A, and L51A, proposed to lie on the interior face of the helix (18), are gain of function mutants. We propose that gain of function is a direct consequence of enhanced monomer formation. The 2.5-fold increase in inhibitory function that accompanies the 3-4-fold enhancement of monomer formation provides kinetic evidence that PLN monomers are the functional inhibitory form, while PLN pentamers represent a noninhibitory or less inhibitory reservoir. In chemical terms, the concentration of PLN monomers, a function of the dissociation constant of the PLN pentamer, will be a key determinant of the concentration of the inhibited PLN monomer·SERCA2a complex ([MS]) and of the degree of SERCA2a inhibition.
We tested the hypothesis that loss of function PLN mutants, with nonfunctional substitutions at specific sites that are essential for interaction with SERCA2a, would be dominant over gain of function mutants, activated by an increase in their monomer concentration, by creating double mutants. In agreement with our hypothesis, the double mutants N34A/I40A and F35A/L44A were found to be virtually inactive (Fig. 1A), even though both were monomeric (Fig. 1C). The mutants form monomers, they may form PLN monomer·SERCA2a complexes, but inhibitory interactions clearly do not occur in the complexes.
Although loss of PLN function is likely to result from disruption of interactions between amino acids in transmembrane helices of PLN and SERCA2a, as proposed, it is also possible that it could result from pentamer stabilization. Data in Table I illustrate that the loss of function mutants, N34A, I38A, V49A and L52A, formed slightly more stable pentamers, in line with their loss of function. The mutant I45A also formed more stable pentamers, however, and function was not altered in this case. These discrepancies will, no doubt, be clarified by further study of the subtleties of the formation of the PLN monomer·SERCA2a complex and the formation of inhibitory interactions within the complex.
Gain of inhibitory function might occur through transmembrane or
cytoplasmic interaction sites or both. Gain of function was first
observed in studies involving coexpression of SERCA2a with PLN domain
II constructs to which NH2-terminal, cytoplasmic epitopes such as PLN1-20 or hemagglutinin were fused (8). An
examination of Fig. 2B in Ref. 8 shows that
PLN1-20-PLN30-52 was a Class 3 mutant that
gained function, while Flag-PLN25-52 was a Class 2 mutant
with unaltered function. These observations suggest that at least one
form of gain of function results from PLN domain II interactions with
SERCA2a. As a further test of this hypothesis, we constructed the
Met-PLN28-52 mutant, L37A, to increase the probability
that this simple domain II construct would be maximally monomeric, in
line with structural predictions for the PLN transmembrane domain. The
L37A mutation enhanced the inhibitory properties of
Met-PLN28-52 from KCa of about
0.17 pCa units (8) to
KCa of about
0.3 pCa units (data not shown). We conclude that gain of inhibitory function can be accomplished with only the transmembrane domain of PLN,
but we recognize that other means of increasing the concentration of
the inhibited PLN monomer·SERCA2a complex would also result in gain
of function.
Fig. 3 illustrates our model for PLN interaction with SERCA2a. We deduce that PLN monomers (M) are the functional species and that their dissociation from pentamers (P) is an essential step in SERCA2a (S) inhibition by PLN. The fact that PLN is about 25% depolymerized under normal conditions implies that PLN monomers are normally in relatively abundant supply. The dissociation constants for both the PLN pentamer (Kd1) and the PLN monomer/SERCA2a heterodimer (Kd2) will control both the PLN monomer concentration [M] and the concentration of the monomer-inhibited form of SERCA2a [MS], defined as follows.
![]() |
(Eq. 1) |
![]() |
(Eq. 2) |
Our observations have important physiological and medical relevance. If mutations of Phe32, Ile33, Leu37, Ile40, Leu43, Leu44, Ile47, Met50, and Leu51 were to occur naturally, we predict that they would increase [MS] to levels that would inhibit Ca2+ removal from the cytoplasm of myocardial cells. Although other Ca2+ removal systems might compensate (24), we predict that the resulting disruption in Ca2+ regulation would lead to cardiomyopathy. We also predict that any mutations in the PLN transmembrane helix, in SERCA2a transmembrane helices, or elsewhere in PLN or SERCA2a, that would increase the affinity between M and S, would also increase [MS], leading to gain of PLN inhibitory function and to cardiomyopathy.
We thank Dr. Robert G. Johnson, Merck Research Laboratories, for his interest and advice and for the gift of the anti-PLN antibody 1D11; Dr. Kevin P. Campbell, University of Iowa, for the gift of the anti-SERCA2a antibody IID8F6; Dr. R. J. Kaufman, Genetics Institute, for the gift of the pMT2 vector; Dr. R. Kopito, Stanford University, for the gift of HEK-293 cells; and Stella de Leon for oligonucleotide synthesis.