THE NUCLEAR MEMBRANE-ASSOCIATED HONEYCOMB STRUCTURE OF THE UNICELLULAR ORGANISM AMEBA-PROTEUS - ON THE SEARCH FOR HOMOLOGIES WITH THE NUCLEAR LAMINA OF METAZOA

In the protozoon Amoeba proteus, a complex and highly organized struct ure with the morphology of a honeycomb is associated with the nucleopl asmic surface of the nuclear membrane. We have tested whether this str ucture exhibits similarity to the nuclear lamina of metazoic organisms , First, we have shown that the honeycomb layer is composed of 3 to 5 nm thick protein fibrils resistant to treatment with detergent, high s alt, and digestion with nucleases, thus possessing properties typical for karyoskeletal elements. However, in contrast to the meshwork of la min filaments in somatic cells of metazoic organisms, the honeycomb la yer is not tightly anchored to the nucleoplasmic side of pore complexe s, or to the inner nuclear membrane. Second, in microinjection experim ents we investigated whether fluorescently labeled lamins of Xenopus l aevis (lamins A and L(I)) and Drosophila melanogaster (lamin Dmo) were able to associate in vivo with the Amoeba proteus honeycomb structure . In microinjected amoeba these three lamins were efficiently transpor ted into the nucleus, but did not associate with the nuclear envelope. Our results suggest that the Amoeba proteus nuclear envelope, includi ng the honeycomb layer, does not contain proteins exhibiting high homo logies to lamins of metazoan species thus preventing the localized ass embly of microinjected lamins along the nuclear periphery.