Y. Kimura et al., PHOSPHOLAMBAN INHIBITORY FUNCTION IS ACTIVATED BY DEPOLYMERIZATION, The Journal of biological chemistry, 272(24), 1997, pp. 15061-15064
Phospholamban (PLN), a homopentameric, integral membrane protein, reve
rsibly inhibits cardiac sarcoplasmic reticulum Ca2+-ATPase (SERCA2a) a
ctivity through intramembrane interactions, Here, alanine-scanning mut
agenesis of the PLN transmembrane sequence was used to identify two fu
nctional domains on opposite faces of the transmembrane helix. Mutatio
ns in one face diminish inhibitory interactions with transmembrane seq
uences of SERCA2a, but have relatively little effect on the pentameric
state, while mutations in the other face activate inhibitory interact
ions 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 int
eraction site lies on the helical face which is not involved in pentam
er 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 PL
N 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 concentra
tion of this inhibited complex is determined by the dissociation const
ant for the PLN pentamer (which is mutation-sensitive) and by the diss
ociation constant for the PLN/SERCA2a heterodimer (which is likely to
be mutation-sensitive).