Nuclear-envelope nucleoside triphosphatase: stimulation by poly(A) (+)mRNA and modulation by microtubule protein.

Abstract

Nuclear envelopes contain a nucleoside triphosphatase which is thought to be involved in the supply of energy for nucleo-cytoplasmic RNA transport. This enzyme is stimulated most efficiently by poly(A) and to a lesser extent by poly(G) and poly(dT). Half-maximal stimulation of the enzyme from rat liver nuclei, which was associated with the poly(A)-specific Endoribonuclease IV and was free from poly(A) polymerase and Endoribonuclease V activity, was determined to occur at a concentration of 1.1 X 10(6) poly(A) molecules/nuclear ghost. Double-reciprocal plot analysis revealed a 2.8-fold stimulation of the enzyme by poly(A). Poly(A) in the hybrid form had no influence on the activity of the nucleoside triphosphatase. Stimulation by oligo(A) required a minimal chain length of 18 nucleotide units. Naturally occurring RNA species enhanced the nucleoside triphosphatase activity, provided they contained a poly(A) segment. Using poly(A)(+)mRNA, half-maximal stimulation was determined to proceed at 0.5 X 10(6) molecules/nuclear ghost. Removal of the poly(A) segment from mRNA mRNA abolished the stimulatory effect on the enzyme. Microtubule protein was found to inhibit the nucleoside triphosphatase efficiently. At a concentration of 2.0 mg/ml, polymerized microtubule protein reduced the enzyme activity by 96%. Dimeric tubulin was less inhibitory, while actin was without any significant effect. From these findings it is suggested that a possible nucleoside triphosphatase-mediated transport of poly(A)(+) mRNA through nuclear envelope is controlled firstly, by the poly(A) segment of this RNA species and secondly, by cytoplasmic microtubules.