  It is shown how the topological string amplitudes encode the BPS structure of
wrapped M2 branes in M-theory compactification on Calabi-Yau threefolds. This
in turn is related to a twisted supersymmetric index in 5 dimensions which
receives contribution only from BPS states. The spin dependence of BPS states
in 5 dimensions is captured by the string coupling constant dependence of
topological string amplitudes.
