6533b7defe1ef96bd1275eab

RESEARCH PRODUCT

Modulation of the nuclear-envelope nucleoside triphosphatase by poly(A)-rich mRNA and by microtubule protein.

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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.