Tryptophan transport in Neurospora crassa: a tryptophan-binding protein released by cold osmotic shock.

Osmotic shock treatment of germinated conidia of Neurospora reduced the capacity for tryptophan transport in these cells approximately 90% without an appreciable loss of cell viability. Tryptophan-binding proteins and alkaline phosphatase were consistently released into the osmotic shock fluid by this treatment. Four lines of evidence suggest that the binding protein may be related to the tryptophan transport system. (i) It appears to be located on or near the cell surface. (ii) a decreased capacity for binding tryptophan was observed in shock fluids from cells repressed for tryptophan uptake; reduced or altered binding capacity was released from a transport-negative mutant. (iii) The specificity of tryptophan binding was similar to that observed in the in vivo transport system. (iv) The dissociation constant for binding, as measured by equilibrium dialysis, was approximately the same as the K(m) for tryptophan transport.