2-MATRIX STRING MODEL AS CONSTRAINED (2-DIMENSIONAL INTEGRABLE SYSTEM(1))

We show that the 2-matrix string model corresponds to a coupled system of 2 + 1-dimensional KP and modified KP ((m)KP2+1) integrable equatio ns subject to a specific ''symmetry'' constraint. The latter together with the Miura-Konopelchenko map for (m)KP2+1 are the continuum incarn ation of the matrix string equation. The (m)KP2+1 Miura and Backlund t ransformations are natural consequences of the underlying lattice stru cture. The constrained (m)KP2+1 system is equivalent to a 1 + 1-dimens ional generalized KP-KdV hierarchy related to graded SL(3,1). We provi de an explicit representation of this hierarchy, including the associa ted W(2,1)-algebra of the second Hamiltonian structure, in terms of fr ee currents.