The proline-rich COOH-terminal region of dynamin binds various Src homology 3 (SH3) domain-containing proteins, but the physiological role of these interactions is unknown. In living nerve terminals, the function of the interaction with SH3 domains was examined. Amphiphysin contains an SH3 domain and is a major dynamin binding partner at the synapse. Microinjection of amphiphysin's SH3 domain or of a dynamin peptide containing the SH3 binding site inhibited synaptic vesicle endocytosis at the stage of invaginated clathrin-coated pits, which resulted in an activity-dependent distortion of the synaptic architecture and a depression of transmitter release. These findings demonstrate that SH3-mediated interactions are required for dynamin function and support an essential role of clathrin-mediated endocytosis in synaptic vesicle recycling.

Facilitated, "cooperative" binding of GAL4-AH to nucleosomal DNA occurred in response to inhibition from the core histone amino termini. The binding of GAL4-AH (which contains the DNA-binding and dimerization domains of GAL4) to nucleosome cores containing multiple binding sites initiated at the end of a nucleosome core and proceeded in a cooperative manner until all sites were occupied. However, following tryptic removal of the core histone amino termini, GAL4-AH binding appeared to be noncooperative, similar to binding naked DNA. Binding of GAL4-AH to nucleosomes bearing a single GAL4 site at different positions indicated that inhibition of GAL4 binding was largely mediated by the histone amino termini and primarily occurred at sites well within the core and not near the end. When the histone amino termini were intact, binding of GAL4-AH to sites near the center of a nucleosome core was greatly enhanced by the presence of additional GAL4 dimers bound to more-accessible positions. These data illustrate that the binding of a factor to more-accessible sites, near the end of a nucleosome, allows facilitated binding of additional factors to the center of the nucleosome, thereby overcoming repression from the core histone amino termini. This mechanism may contribute to the binding of multiple factors to complex promoter and enhancer elements in cellular chromatin.

The binding of a GAL4-AH, USF or Sp1 to nucleosome cores was stimulated by the presence of the histone-binding protein, nucleoplasmin. Stimulation of factor binding by nucleoplasmin was specific for nucleosome reconstituted DNA and was not mimicked by non-specific histone sinks (i.e. polyglutamate or RNA). Upon GAL4-AH binding, nucleoplasmin specifically removed histones H2A and H2B from the nucleosome which enhanced the subsequent loss of the H3/H4 tetramers onto competing DNA. Thus, nucleoplasmin participated in the complete conversion of nucleosome cores to transcription factor-DNA complexes. These data indicate that proteins which bind histones can increase transcription factor binding to nucleosomal DNA and that transcription factor binding can initiate nucleosome disassembly. Similar activities of histone-binding proteins may participate in the displacement of nucleosomes at enhancers and promoters in vivo.