Quantitative determination of nuclear pore complexes in cycling cells with differing DNA content

Abstract

The number of pore complexes per nucleus was determined for a wide variety of cultured cells [Xenopus laevis kidney A6 and heart cells, mouse L929 cells, African Green Monkey CU1 cells, Peromyscus crinitus cells, Rana pipiens cells, Notophthalmus viridescens cells, Drosophila S2 cells, Triturus cristatus heart cells, Scaphiopus holbrooki heart cells, human cervical cancer HeLa cells, WI38 cells, Chinese hamster ovary cells, Saccharomyces cerevisiae] selected for their variable DNA content over a range of 1-5600. The pore number was compared to DNA content, nuclear surface area and nuclear volume. Values for pore frequency (pores/.mu.m2) were relatively constant in the species studied. When the pore to DNA ratio was plotted against the DNA content, there was a remarkable correlation which decreased exponentially for the cells of vertebrate origin. Exceptions were the heteroploid mammalian cells which had the same ratio as the diploid mammalian cells despite higher DNA content. The results are interpreted to mean that neither the nuclear surface, the nuclear volume, nor the DNA content alone determines the pore number of the nucleus, but rather an as yet undetermined combination of different factors. The surface and the volume of vertebrate nuclei do not decrease with decreasing DNA content below a given value. The following speculation is suggested to account for the anomalous size changes of the nucleus relative to DNA content in vertebrates. Species with small DNA complements have a relatively large proportion of active chromatin which determines the limits of the physical parameters of the nucleus. The amount of active chromatin may be the same for at least the vertebrates with low DNA content. At high DNA content, the nuclear parameters may be determined by relatively high proportion of inactive condensed chromatin which increases the nuclear surface and volume.