  Extra neutral gauge bosons and Higgs bosons in an effective low-energy
$SU(2)_L \times SU(2)_I \times U(1)_Y \times U(1)_{Y^{\prime}}$ model, which is
a subgroup of $E_6$, are studied. $SU(2)_I$ is a subgroup of $SU(3)_R$ and
commutes with the electric charge operator, so the three corresponding gauge
bosons are neutral. Electroweak precision experiments are used to put
constraints on masses of the extra neutral gauge bosons and on the mixings
between them and the ordinary Z boson, including constraints arising from a
proposed measurment of the weak charge of the proton at Jefferson Lab. Bounds
on and relationships of masses of Higgs bosons in the supersymmetric version of
the model are also discussed.
