Modulation of the nuclear-envelope nucleoside triphosphatase by poly(A)-rich mRNA and 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 × 106 poly(A) molecules/nuclear ghost. Double‐reciprocal plot analyses 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 minimum chain length of 18 nucleotide units. Naturally occurring RNA species enhanced the nucleoside triphosphatase activity, provided that they contained a poly(A) segment. Using poly(A)‐rich mRNA, half‐maximal stimulation was determined to proceed at 0.5 × 106 molecules/nuclear ghost. Removal of the poly(A) segment from 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)‐rich mRNA through nuclear envelopes is controlled, first, by the poly(A) segment of this RNA species and, secondly, by cytoplasmic microtubules.