Protein import, nucleus types

Figure 12.8 Model for general and cell-specific plasmid nuclear import. (A) SV40 enhancer-mediated nuclear import. Because the transcription factors bound by this DNA sequence are ubiquitously expressed, SV40 DNA localizes to the nuclei of all cell types (see Table 12.1). (B) Smooth muscle-specific plasmid nuclear import. Smooth muscle-specific transcription factors, including SRF among others, can bind to their target sites within the SMGA promoter carried on a plasmid and serve to transport the DNA to the nucleus via interactions with the NLS-mediated protein import machinery. Since these factors are not expressed in other cell types, no nuclear import will occur in non-smooth muscle cells.

The acetylation and deacetylation of histones figure prominently in the processes that activate chromatin for transcription. As noted above, the amino-terminal domains of the core histones are generally rich in Lys residues. Particular Lys residues are acetylated by histone acetyltransferases (HATs). Cytosolic (type B) HATs acetylate newly synthesized histones before the histones are imported into the nucleus. The subsequent assembly of the histones into chromatin is facilitated by additional proteins CAF1 for H3 and H4, and NAP1 for H2A and H2B. (See Table 28-2 for an explanation of some of these abbreviated names.)... 

---