Reversible acetylation of the N-terminal tails of histones plays a key role in the regulation of various nuclear activities such as chromatin assembly, replication, and transcription (2
). The acetylation of lysine residues within nucleosomes weakens the interaction of the histone tails with the DNA and leads to chromatin decompaction (16
). These structural transitions enhance the accessibility of the underlying DNA sequence to various factors, thereby reducing the repressive effect of the nucleosome on transcription and replication. The relationship between transcriptional regulation and histone acetylation has been strengthened considerably by the discovery that certain factors associated with transcriptional activation have intrinsic histone acetylase activity (7
), while factors associated with transcriptional repression contain histone deacetylase activity (26
). It is significant that in some cases this reversible acetylation is targeted and specific. For example, Tetrahymena
GCN5 preferentially acetylates residues K8 and K16 of histone H4 and K14 of histone H3 (13
). In contrast, in Saccharomyces cerevisiae
, transcriptional repression by UME6 involves the specific deacetylation of K5 in histone H4 by the deacetylase RPD3 (40
). Furthermore, the pattern of H4 acetylation in heterochromatin is unique, suggesting that specific acetylation marks discrete functional states of chromatin structure (5
). Taken together with other findings, these results suggest that the reversible acetylation of histones is not merely a mechanism for indiscriminately unfolding chromatin but is a key step in the selective regulation of the expression of specific genes.
Most of the studies on the effect of acetylation on the structure and function of chromatin have focused on the reversible acetylation of the core histones. However, other chromatin-associated proteins, such as the nonhistone HMG proteins (9
), are also reversibly modified (23
). In duck erythrocytes, two acetylation sites in HMG-1 and HMG-14 and three sites in HMG-17 were identified (42
). In the HMG-14/-17 protein family the major acetylation site detected even in cells not treated with deacetylase inhibitors is the lysine at position 2 (43
). The enzymes responsible for the reversible acetylation of HMG proteins have not been identified, and nothing is known about the functional consequences of HMG acetylation.
Chromosomal proteins HMG-14 and HMG-17 are the only nuclear proteins known to specifically bind to the 146-bp nucleosome core particle and therefore could be considered as an integral part of the chromatin fiber (9
). These HMG proteins specifically interact with the N termini of the core histones (11
) and produce distinct footprints on the nucleosome core (1
). Removal of the N termini of the core histones greatly reduces the binding of the proteins to the nucleosome core (11
). By site-directed cross-linking we demonstrated that HMG-14 contacts the nucleosome at multiple sites (47
). The N terminus of HMG-14 specifically interacts with histone H2B, while the C terminus of the protein specifically interacts with the N terminus of histone H3. In chromatin, HMG-containing nucleosomes are clustered into distinct domains, which on the average consist of six contiguous nucleosome-HMG complexes (35
). The binding of HMG-14/-17 proteins to nucleosomes unfolds the higher-order chromatin structure and enhances various DNA-dependent activities, such as transcription (12
) and replication (50
A mechanistic view of these findings suggests that HMG-14/-17 proteins unfold the higher-order chromatin structure, thereby promoting access to nucleosomes by various regulatory factors, some of which may have histone acetyltransferase (HAT) activity. In view of the close proximity between the acetylation sites in histones (histone tails) and HMG-14/-17 proteins (47
) and the recent observation that some HATs can modify transcription factors (22
), it is plausible that these enzymes could also modify HMGs.
Here we report that PCAF specifically acetylates HMG-17 but not HMG-14, and we demonstrate that the specificity of acetylation may require a distinct protein conformation. We examine the ability of PCAF to acetylate HMG-nucleosome complexes and demonstrate that the presence of HMG-14/-17 proteins affects the rate of acetylation of the N termini of core histones. We studied the effect of acetylation on the interaction of HMG-17 with nucleosomes and show that acetylation of HMG-17 affects its interaction with nucleosomes. These findings represent the first identification of an acetylase capable of specifically acetylating a nonhistone structural chromosomal protein and provide insight into a mechanism whereby HATs affect transcription in the context of chromatin.