It is by now widely recognized that cell membranes show complex patterns of lateral organization. Two mechanisms involving either a lipid-dependent (microdomain model) or cytoskeleton-based (meshwork model) process are thought to be responsible for these plasma membrane organizations. In the present study, fluorescence correlation spectroscopy measurements on various spatial scales were performed in order to directly identify and characterize these two processes in live cells with a high temporal resolution, without any loss of spatial information. Putative raft markers were found to be dynamically compartmented within tens of milliseconds into small microdomains (+o120 nm) that are sensitive to the cholesterol and sphingomyelin levels, whereas actin-based cytoskeleton barriers are responsible for the confinement of the transferrin receptor protein. A free-like diffusion was observed when both the lipid-dependent and cytoskeleton-based organizations were disrupted, which suggests that these are two main compartmentalizing forces at work in the plasma membrane.

The Mll gene is a member of the mammalian trithorax group, involved with the antagonistic Polycomb group in epigenetic regulation of homeotic genes. MLL contains a highly conserved SET domain also found in various chromatin proteins. In this study, we report that mice in which this domain was deleted by homologous recombination in ES cells (⌬SET) exhibit skeletal defects and altered transcription of particular Hox genes during development. Chromatin immunoprecipitation and bisulfite sequencing analysis on developing embryo tissues demonstrate that this change in gene expression is associated with a dramatic reduction in histone H3 Lysine 4 monomethylation and DNA methylation defects at the same Hox loci. These results establish in vivo that the major function of Mll is to act at the chromatin level to sustain the expression of selected target Hox genes during embryonic development. These observations provide previously undescribed evidence for the in vivo relationship and SET domain dependence between histone methylation and DNA methylation on MLL target genes during embryonic development.histone methyltransferase ͉ MLL-SET domain ͉ homeotic transformations T he control of cell identity during development is specified, in large part, by the unique expression patterns of multiple homeobox-containing (Hox) genes in specific segments of the embryo (1). The trithorax and polycomb groups (trx-G and PcG) were identified for their role in faithfully maintaining the transcriptional states of these key developmental regulators, providing an epigenetic mechanism of cellular memory (2-4).The gene expression maintenance function of the trxG and PcG proteins is highly conserved. Mixed lineage leukemia (Mll), a human homolog of Drosophila trithorax and a member of the trxG family, was identified first for its involvement in chromosomal translocations associated with lymphoid and myeloid acute leukemia in infants and adults (5, 6). Mll encodes a 3,969-aa nuclear protein with multiple domains, including three AT-hook motifs, a DNA methyltransferase homology domain (DNMT) in the aminoterminal half of the protein, a central zinc finger (PHD) region, and a highly conserved 130-aa carboxyl-terminal SET domain. The MLL protein was shown to be proteolytically processed into two portions (MLL N and MLL C ) with antagonistic transcriptional effector properties, that reassociate and stabilize each other (7-9). The MLL protein is critical for proper regulation of the Hox genes during embryonic development (10). In Mll null mutant mice (MllϪ͞Ϫ), Hox gene expression is correctly initiated but is not sustained as the function of Mll becomes necessary (11), leading to embryonic lethality.It is strongly believed that maintenance of the transcriptional status of target genes by PcG and trxG proteins is achieved through chromatin modifications (12). The structure similarity between some trxG͞PcG and suppressors or enhancers of position effect variegation (PEV) further substantiates this point. One of the most remarkable shared domains within th...