This a transmembrane helix (the same as the previous exercise). The 'positive
in' rule tells us that TSKKT is cytosolic. The sequence NHS thus must be the
extra-cellular bit that can trigger the immune system. So we need to mutate
there. H->Q could be nice, but obviously hundreds of other mutations at
any of the positions NHS are OK.
However, mutations that include a special residue (Trp, Gly, Pro, Cys, Etc)
are not so good because we want to loose function (antigenicity), and not gain
another function.
You get extra points if you mention that the S also cannot be mutated because it is important for the helix stability at this position (one before the helix).