  We discuss, for the first time, the role of the superpotential couplings of
three messenger fields in a GMSB-type unification model in which messenger
sector consists of both 5+5bar and 10+10bar of SU(5). It turns out that these
interactions are relevant when coexist with appropriate messenger-MSSM
couplings. Then they induce extra contributions to 2-loop soft masses. In the
present model, we derive all such soft terms and discuss constraints which have
to be satisfied to avoid rapid proton decay and mu/B_mu problem. As an example,
it is shown how superpotential couplings of three messengers influence mass
spectra when the model is restricted by additional global U(1)_q symmetry. We
find that masses of the lightest sleptons are the most sensitive to those new
interactions, what in some cases results in the NLSP/NNLSP pattern in which
stau or selectron is lighter than the lightest neutralino.
