The kinetics of l-aspartate transport in Neurospora crassa conidia

Abstract

Transport of the acidic amino acids by Neurospora crassa conidia is shown to be mediated by neutral and general amino acid transport systems. The neutral transport system is shown to possess “high affinity”, but low transport capacity for the acidic amino acids. The general transport system is shown to have “low affinity” and high transport capacity for these amino acids. The non-linearity of a Lineweaver-Burk plot of aspartate transport by the wild type has been shown to result from an overlapping of two transport systems for accumulation of this amino acid. Removal, by mutation, of the neutral transport system restores the transport kinetics to a linear function. The combined, and separate, transport activities are shown to be stereospecific, preferring the l-stereoisomeric form over the d-form, and to transport the neutrally charged species of l-aspartate and l-glutamate. The transport activity is dependent on pH, and the V, but not the C 1 2 , constant of the general transport system is pH dependent. Two models for amino acid transport are presented as possible mechanisms to explain the observed kinetics. One model involves three genetically distinct transport systems. The second model utilizes a single “allosteric permease” to account for the kinetic data.