Nucleocytoplasmic translocation of
polyadenylated messenger RNA is an energy-dependent process which is regulated by a nuclear envelope
nucleoside triphosphatase; this
enzyme was found to be stimulated by the 3'-terminal polyadenylic
acid [
poly(A)] tail of
messenger RNA (Bernd, A., Schröder, H. C., Zahn, R. K., and Müller, W. E. G. Eur. J. Biochem., 129: 43-49, 1982).
RNA efflux from isolated mouse
lymphoma (L5178Y) cell nuclei is strongly reduced if
9-beta-D-arabinofuranosyladenine 5'-triphosphate (
ara-ATP) is present in the transport medium. Half-maximal inhibition of
RNA efflux occurs with 120 microM
ara-ATP. Most likely, the inhibitory effect of
ara-ATP is caused by inhibition of nuclear envelope
nucleoside triphosphatase; this
enzyme was found to be highly sensitive to inhibition by this
antibiotic. The inhibition type of the
nucleoside triphosphatase of rat liver nuclear ghosts is competitive with respect to
ATP; the Ki:Km ratio was determined to be 0.27. Besides
nucleoside triphosphatase, nuclear envelopes contain a
protein phosphokinase modulating the affinity of pore complex laminae to
poly(A). This
enzyme was also found to be strongly inhibited by
ara-ATP in a competitive way with respect to
ATP (Ki:Km, 0.056) and could therefore also contribute to the overall inhibition of RNA transport. The polyadenylation of endogenous
RNA by
poly(A) polymerase(s) in intact rat liver nuclei as well as in nuclear matrices isolated from the same source was found to be markedly suppressed in the presence of
ara-ATP. The inhibitions of both
poly(A) polymerase activities (contained in whole nuclei or nuclear matrix bound) are of the competitive type with respect to
ATP. In in vitro assays, nuclear envelope
nucleoside triphosphatase is inhibited by microtubule
protein. Of the 2
ATP-dependent
enzyme activities associated with microtubule
protein (cyclic
adenosine 3':5'-monophosphate-dependent
protein kinase and
adenosine triphosphatase), only the
kinase was slightly affected by
ara-ATP. Cellular uptake of
adenosine 5'-monophosphate and perhaps
9-beta-D-arabinofuranosyladenine 5'-monophosphate (
ara-AMP) is facilitated by a cellular membrane-bound
5'-nucleotidase. Our studies revealed that neither cleavage of
ara-AMP nor inhibition of the
enzyme activity by
ara-AMP occurs.
9-beta-D-Arabinofuranosyladenine and
ara-AMP represent neither direct
mutagens nor premutagens as determined by the Salmonella-mammalian microsome mutagenicity test.