N. Boxenbaum et al., CHANGES IN STEADY-STATE CONFORMATIONAL EQUILIBRIUM RESULTING FROM CYTOPLASMIC MUTATIONS OF THE NA,K-ATPASE ALPHA-SUBUNIT, The Journal of biological chemistry, 273(36), 1998, pp. 23086-23092
Mutations comprising either deletion of 32 amino acids from the NH2 te
rminus (alpha 1M32) or a Glu(233) --> Lys substitution in the first M2
-M3 cytoplasmic loop (E233K) of the alpha 1-subunit of the Na,K-ATPase
result in a shift in the steady-state E-1 <-> E-2 conformational equi
librium toward E-1 form(s), In the present study, the functional conse
quences of both NH2-terminal deletion and Glu(233) substitution provid
e evidence for mutual interactions of these cytoplasmic regions. Follo
wing transfection and selection of HeLa cells expressing the ouabain-r
esistant alpha 1M32E233K double mutant, growth was markedly reduced un
less the K+ concentration in the culture medium was increased to at le
ast 10 mM. Marked changes effected by this double mutation included 1)
a 15-fold reduction in catalytic turnover (V-max/EPmax) a 70-fold inc
rease in apparent affinity for ATP, 3) a marked decrease in vanadate s
ensitivity, and 4) marked (approximate to 10-fold) K+ activation of th
e Na-ATPase activity measured at micromolar ATP under which condition
the E-2(K) --> --> E-1 pathway is normally (alpha 1) rate-limiting and
K+ is inhibitory. The decrease in catalytic turnover was associated w
ith a 5-fold decrease in V-max and a compensatory approximate to 3-fol
d increase in expressed alpha 1M32E233K protein. In contrast to the be
havior of either alpha 1M32 or E233K, alpha 1M32E233K also showed alte
rations in apparent cation affinities. K-Na' was decreased approximate
to 2-fold and K-K' was increased approximate to 2-fold. The importanc
e of the charge at residue 233 is underscored by the consequences of s
ingle and double mutations comprising either a conservative change (E2
33D) or neutral substitution (E233Q). Thus, whereas mutation to a posi
tively charged residue (E233K) causes a drastic change in enzymatic be
havior, a conservative change causes only a minor change and the neutr
al substitution, an intermediate effect. Overall, the combined effects
of the NH2-terminal deletion and the Glu(233) substitutions are syner
gistic rather than additive, consistent with an interaction between th
e NH2-terminal region, the first cytoplasmic loop, and possibly the la
rge M4-M5 cytoplasmic loop bearing the nucleotide binding and phosphor
ylation sites.