Csi1 promotes centromere clustering by linking centromeres to the SUN domain protein Sad1 in the nuclear envelope.

Heterochromatin spreading leads to the silencing of genes within its path, and boundary elements have evolved to constrain such spreading. In fission yeast, heterochromatin at centromeres I and III is flanked by inverted repeats termed IRCs, which are required for proper boundary functions. However, the mechanisms by which IRCs prevent heterochromatin spreading are unknown. Here, we identified Bdf2, which is homologous to the mammalian bromodomain and extraterminal (BET) family double bromodomain proteins involved in diverse types of cancers, as a factor required for proper boundary function at IRCs. Bdf2 is enriched at IRCs through its interaction with the boundary protein Epe1. The bromodomains of Bdf2 recognize acetylated histone H4 tails and antagonize Sir2-mediated deacetylation of histone H4K16. Furthermore, abolishing H4K16 acetylation (H4K16ac) with an H4K16R mutation promotes heterochromatin spreading, and mimicking H4K16ac by an H4K16Q mutation blocks heterochromatin spreading at IRCs. Our results thus illustrate a mechanism of establishing chromosome boundaries at specific sites through the recruitment of a factor that protects euchromatic histone modifications. They also reveal a previously unappreciated function of H4K16ac in cooperation with H3K9 methylation to regulate heterochromatin spreading.

Background: Histone acetylation regulates diverse cellular processes. Results: The fission yeast Mst2 complex is a specific histone H3 lysine 14 acetyltransferase. Conclusion: H3K14 acetylation is required for DNA damage checkpoint activation. Significance: These analyses define the in vivo functions of the acetylation of a single histone lysine residue.

Asp-His-His-Cys (DHHC) cysteine-rich domain (CRD)acyltransferases are polytopic transmembrane proteins that are found along the endomembrane system of eukaryotic cells and mediate palmitoylation of peripheral and integral membrane proteins. Here, we address the in vivo substrate specificity of five of the seven DHHC acyltransferases for peripheral membrane proteins by an overexpression approach. For all analysed DHHC proteins we detect strongly overlapping substrate specificity. In addition, we now show acyltransferase activity for Pfa5. More importantly, the DHHC protein Pfa3 is able to trap several substrates at the vacuole. For Pfa3 and its substrate Vac8, we can distinguish two consecutive steps in the acylation reaction: an initial binding that occurs independently of its central cysteine in the DHHC box, but requires myristoylation of its substrate Vac8, and a DHHC-motif dependent acylation. Our data also suggest that proteins can be palmitoylated on several organelles. Thus, the intracellular distribution of DHHC proteins provides an acyltransferase network, which may promote dynamic membrane association of substrate proteins.

Vacuole biogenesis depends on specific targeting and retention of peripheral membrane proteins. At least three palmitoylated proteins are found exclusively on yeast vacuoles: the fusion factor Vac8, the kinase Yck3, and a novel adaptor protein implicated in microautophagy, Meh1. Here, we analyze the role that putative acyltransferases of the DHHC family play in their localization and function. We find that Pfa3͞Ynl326c is required for efficient localization of Vac8 to vacuoles in vivo, while Yck3 or Meh1 localization is not impaired in any of the seven DHHC deletions. Vacuoleassociated Vac8 appears to be palmitoylated in a pfa3 mutant, but this population is refractive to further palmitoylation on isolated vacuoles. Vacuole morphology and inheritance, which both depend on Vac8 palmitoylation, appear normal, although there is a reduction in vacuole fusion. Interestingly, Pfa3 is required for the vacuolar localization of not only an SH4 domain that is targeted by myristate͞palmitate (as in Vac8) but also one that is targeted by a myristate͞basic stretch (as in Src). Our data indicate that Pfa3 has an important but not exclusive function for Vac8 localization to the vacuole.Yck3 ͉ SH4 domain ͉ acylation ͉ membrane targeting P rotein and lipid trafficking along the endomembrane system occurs by vesicular transport (1). Of all proteins implicated in vesicle fusion and fission, only a subset is permanently associated with membranes via transmembrane segments, whereas most are recruited to the membrane from the cytoplasm. The latter proteins depend on membrane receptors, lipids, or lipid anchors for binding to their appropriate target membrane (2). This poses the question of how the recruitment of these proteins is coordinated with their function.Palmitoylation has been discussed as a special lipid modification. It may direct proteins to specific membrane domains (3-5), and it is the only common lipid modification that is reversible, permitting cycling of a protein between membranes and cytosol. The identification and characterization of the underlying acylation͞deacylation machinery is therefore critical for understanding the function of palmitoylation. Recently, biochemical and genetic analyses have identified several proteins that are required for palmitoylation, including those of the so-called DHHC-CRD family of polytopic membrane proteins (6). Yeast contains seven homologs (Erf2, Swf1, Yol003c, Akr1, Akr2, Ydr459c, and Ynl326c͞Pfa3), which seem to be distributed throughout the endomembrane system as suggested by the GFP database and recent studies. At the ER, Swf1 is required for palmitoylation of Tlg1 (7), and Erf2 promotes Ras2 palmitoylation (8, 9). The Golgi-localized Akr1 is responsible for palmitoylation of the casein kinase I (CKI) isoform Yck2 (10). Similarly, several of the 20 or so mammalian DHHC proteins localize to distinct organelles, with critical roles for the palmitoylation of PSD-95, Ras, and SNAP-25 (11-13). This wide distribution of proteins involved in palmitoylation suggests that palmitoylati...

No abstract

Palmitoylation stably anchors specific proteins to membranes, but may also have a direct effect on the function of a protein. The yeast protein Vac8 is required for efficient vacuole fusion, inheritance and cytosol-to-vacuole trafficking. It is anchored to vacuoles by an N-terminal myristoylation site and three palmitoylation sites, also known as the SH4 domain. Here, we address the role of Vac8 palmitoylation and show that the position and number of substrate cysteines within the SH4 domain determine the vacuole localization of Vac8: stable vacuole binding of Vac8 requires two cysteines within the N-terminus, regardless of the combination. Importantly, our data suggest that palmitoylation adds functionality to Vac8 beyond simple localization. A mutant Vac8 protein, in which the palmitoylation sites were replaced by a stretch of basic residues, still localizes to vacuole membranes and functions in cytosol-to-vacuole transport, but can only complement the function of Vac8 in morphology and inheritance if it also contains a single cysteine within the SH4 domain. Our data suggest that palmitoylation is not a mere hydrophobic anchor required solely for localization, but influences the protein function(s).

The dually lipidated SNARE Ykt6 is found on intracellular membranes and in the cytosol. In this study, we show that Ykt6 localizes to the Golgi as well as endosomal and vacuolar membranes in vivo. The ability of Ykt6 to cycle between the cytosol and the membranes depends on the intramolecular interaction of the N-terminal longin and C-terminal SNARE domains and not on either domain alone. A mutant deficient in this interaction accumulates on membranes and -in contrast to the wild-type proteindoes not get released from vacuoles. Our data also indicate that Ykt6 is a substrate of the DHHC (Asp-HisHis-Cys) acyltransferase network. Overexpression of the vacuolar acyltransferase Pfa3 drives the F42S mutant not only to the vacuole but also into the vacuolar lumen. Thus, depalmitoylation and release of Ykt6 are needed for its recycling and to circumvent its entry into the endosomal multivesicular body pathway.