  We propose a modification of the AKNS hierarchy that includes the "modified"
Pohlmeyer-Lund-Regge (mPLR) equation. Similarity reductions of this hierarchy
give the second, third, and fourth Painlev\'e equations. Especially, we present
a new Lax representation and a complete description of the symmetry of the
third Painlev\'e equation through the similarity reduction. We also show the
relation between the tau-function of the mPLR hierarchy and Painlev\'e
equations.
