Calcium modulation of polyamine transport is lost in a putrescine-sensitive mutant of Neurospora crassa

Abstract

Putrescine transport in Neurospora is saturable and concentrative in dilute buffers, but in the growth medium putrescine simply equilibrates across the cell membrane. We describe a mutant, puu-1, that can concentrate putrescine from the growth medium because the polyamine transport system has lost its normal sensitivity to Ca 2+. The wild type closely resembles the mutant if it is washed with citrate and ethylene glycol bis(β-aminoethyl ether) N,N′-tetraacetic acid. The mutant phenotype also appears in the wild type after treatment with cycloheximide. The results suggest that putrescine uptake is normally regulated by an unstable Ca 2+ -binding protein that restricts polyamine uptake. This protein is evidently distinct from the polyamine-binding function for uptake, which is normal in mutant and in cycloheximide-treated wild type cells. The puu-1 mutation, stripping of Ca 2+, and cycloheximide treatment all cause an impairment of amino acid transport, indicating that other membrane transport functions rely upon the product of the puu-1 + gene. Preliminary evidence suggests that the putrescine carrier is not the Ca 2+-sensitive, low-affinity K +-transport system, but K + efflux does accompany putrescine uptake.