M. E. E. Luderus

Binding of matrix attachment regions to lamin polymers involves single-stranded regions and the minor groove.

Chromatin in eukaryotic nuclei is thought to be partitioned into functional loop domains that are generated by the binding of defined DNA sequences, named MARs (matrix attachment regions), to the nuclear matrix. We have previously identified B-type lamins as MAR-binding matrix components (M. E. E. Ludérus, A. de Graaf, E. Mattia, J. L. den Blaauwen, M. A. Grande, L. de Jong, and R. van Driel, Cell 70:949-959, 1992). Here we show that A-type lamins and the structurally related proteins desmin and NuMA also specifically bind MARs in vitro. We studied the interaction between MARs and lamin polymers in molecular detail and found that the interaction is saturable, of high affinity, and evolutionarily conserved. Competition studies revealed the existence of two different types of interaction related to different structural features of MARs: one involving the minor groove of double-stranded MAR DNA and one involving single-stranded regions. We obtained similar results for the interaction of MARs with intact nuclear matrices from rat liver. A model in which the interaction of nuclear matrix proteins with single-stranded MAR regions serves to stabilize the transcriptionally active state of chromatin is discussed.

Modulation of the interaction between chemotactic cAMP-receptor and N-protein by cAMP-dependent kinase in Dictyostelium discoideum membranes

Dictyostelium discoideum cells contain kinetically distinguishable surface cAMP receptors. Both GTP and GDP lower the receptor affinity by inducing conversion of slowly dissociating sites to fast dissociating sites, presumably by binding to a N‐protein [(1985) Mol. Cell. Biochem. 67, 119‐124, and (1986) Biochemistry 25, 1314‐1320]. In this paper we show that treatment of isolated membranes with cAMP‐dependent protein kinase abolished the GTP‐induced receptor transition, but not the one induced by GDP. The effect of GTP on the receptor kinetics could be restored by treatment of the membranes with alkaline phosphatase. These results indicate that in D. discoideum membranes phosphorylation of a signal‐transduction component reversibly abolishes the interaction of the cAMP receptor with the NGTP complex, but not that with the NSDP complex.

Analysis of the ras Gene Function in Dictyostelium Discoideum

The Dictyostelium discoideum genome contains a single ras gene encoding a putative protein related to mammalian ras (1) . This gene is expressed in vegetative cells, growing in the presence of a food source. Upon starvation cell division stops and a specific developmental cycle is induced which leads to the formation of a fruiting body composed of only two parts, stalk and spores. The latter cells can give rise to vegetative cells in the presence of a new food source (for review see 2) . Transcripts from the Dictyostelium ras gene can be detected in vegetatively growing cells and accumulate during late development only in prestalk cells. At that stage two mRNAs of different size can be detected. The ras protein steady state level present in vegetative cells decreases steadily during early development, then more abruptly passed the pseudoplasmodium stage. Studies by others showed that the ras protein is mainly synthesized in vegetative cells and during late development, corresponding to the detection of ras mRNA (3).