Effect of temperature on nuclear membranes and nucleo-cytoplasmic RNA-transport inTetrahymena grown at different temperatures

Abstract

The effect of temperature on the nuclear envelope structure and the transport of total RNA and ribosomal subunits from nucleus to cytoplasm was examined in Tetrahymena cells propagated at two different temperatures. Freeze-etch electron microscopy of cells grown at 23 and 18 degrees C detects the emergence of smooth areas on the fracture faces of the nuclear membranes upon lowering the temperature below approximately 15 and approximately 12 degrees C, respectively. Coincident with these freeze-etch changes, a discontinuous decrease is observed in the nucleocytoplasmic RNA-transport; this is probably not due to a cease in RNA-synthesis. Below the thermotropic discontinuity observed in the transport of total RNA in 18 degrees-cells the nucleocytoplasmic transport of the small and large ribosomal subunits is equally retarded. Recent temperature studies on the endoplasmic reticulum membranes of Tetrahymena suggest that the freeze-etch changes in the nuclear membranes are induced by a thermotropic clustering of the membrane lipids. We conclude that this lipid clustering induces the permanent protein constituents in the nuclear envelope pore complexes to change from a relatively "open" into a relatively "closed" state thus causing the observed decrease in RNA-transport.

Activation energy for RNA transport from isolated rat liver nuclei.

“…This same elevating effect was demonstrable in our assay system, even with RNase inhibitor present, under incubation conditions encouraging a lower percentage of RNA transport than that obtained with ATP (unpublished results). Such considerations suggest a reasonable agreement between our in vitro data and the observations of Nagel and Wunderlich (20). Therefore the stringent control of supernatant RNase activity is crucial to the study of in vitro RNA transport, because the magnitude of the transport component must be very much greater than the contributions of competing processes in order to provide a sound basis for subsequent Arrhenius analysis.…”

“…The loss of such a component may be related to transcription, which, in our procedure, occurred in vivo before isolation and incubation of the nuclei. In agreement with this concept, Nagel and Wunderlich (20) concluded that "nucleocytoplasmic RNA transport is rate-limited at or even before the transcriptional level above the discontinuity" (which they observed). It is also possible that such experiments involved nonlinear results.…”

“…The apparent magnitude of the activation energy derived from the upper-temperature domain in the Tetrahymena system (20) was t15 kcal/mol. This value is somewhat difficult to interpret because RNase activity was present and unopposed in these workers' system, and the amount of degradation is not necessarily insignificant when compared with the transport obtained.…”

“…Because initial linear rates exhibited no discontinuities on Arrhenius graphs, it is possible that the transition that appeared on the Arrhenius graph for in vivo RNA transport in Tetrahymena (20) signified that a component of the transport process had been lost during isolation of the nucleus. The loss of such a component may be related to transcription, which, in our procedure, occurred in vivo before isolation and incubation of the nuclei.…”

In vitro nuclear transport of ribosomal ribonucleoprotein: temperature affects quantity but not quality of exported particles.

“…As early as 1965, Bier (6) showed that in the ovaries of Musca sp., quick cooling dramatically reduces the overall RNP transport, whereas overall RNA synthesis continues, albeit at lower rates, thus causing an accumulation of RNP in the nuclei. Similarly, a decrease in temperature induces a decrease in the nucleocytoplasmic overall RNP transport in whole cells of the unicellular eucaryote Tetrahymena pyriformis (22,38). This cannot be explained as a reduction of the overall RNA transcription; rather, temperature influences a posttranscriptional event.…”