Exogenous histone H1 injection into mitotic cells disrupts synchronous progression of mitotic events by delaying chromosome decondensation

Abstract

ABSTRACT At the end of open mitosis, chromosome decondensation, nuclear envelope re-formation and reassembly of interphase microtubules following mitotic spindle dissociation occur coordinately. To determine whether these events progress only synchronously in vivo, we delayed chromosome decondensation by injecting of exogenous proteins into the mitotic rat kangaroo kidney epithelium (PtK2) cells. When histone H1 purified from calf thymus was injected at prometaphase, chromosome condensation was prolonged for several hours, and sister chromatid separation and cytokinesis did not occur. However, interphase microtubules reassembled and lamin B-positive structures re-formed around the condensed chromosomes. Exactly the same results were obtained on injection of bacterially expressed H1. Kinetic experiments showed that there were two types of lamin B-positive structures. One type (type A) was stained uniformly with anti-lamin B antibodies. The other (type B) showed peripheral lamin B staining; that is, the normal interphase staining pattern, and was found to be competent for nuclear protein transport. As the chromosomes decondensed, the amount of type A decreased and that of type B increased. However, even cells containing highly condensed chromosomes had both type A and type B. From these results, we conclude that the re-formation of microtubules and reassembly of a nuclear transport-competent envelope do not depend on chromosome decondensation.