Assembly of silent chromatin domains in budding yeast involves the deacetylation of histone tails by Sir2 and the association of the Sir3 and Sir4 proteins with hypoacetylated histone tails. Sir2 couples deacetylation to NAD hydrolysis and the synthesis of a metabolite, O-acetyl-ADP-ribose (AAR), but the functional significance of NAD hydrolysis or AAR, if any, is unknown. Here we examine the association of the Sir2, Sir3, and Sir4 proteins with each other and histone tails. Our analysis reveals that deacetylation of histone H4-lysine 16 (K16), which is critical for silencing in vivo, is also critical for the binding of Sir3 and Sir4 to histone H4 peptides in vitro. Moreover, AAR itself promotes the association of multiple copies of Sir3 with Sir2/Sir4 and induces a dramatic structural rearrangement in the SIR complex. These results suggest that Sir2 activity modulates the assembly of the SIR complex through both histone deacetylation and AAR synthesis.

Silent chromatin domains in Saccharomyces cerevisiae represent examples of epigenetically heritable chromatin. The formation of these domains involves the recruitment of the SIR complex, composed of Sir2, Sir3, and Sir4, followed by iterative cycles of NAD-dependent histone deacetylation and spreading of SIR complexes over adjacent chromatin domains. We show here that the conserved bromo-adjacent homology (BAH) domain of Sir3 is a nucleosome- and histone-tail-binding domain and that its binding to nucleosomes is regulated by residues in the N terminus of histone H4 and the globular domain of histone H3 on the exposed surface of the nucleosome. Furthermore, using a partially purified system containing nucleosomes, the three Sir proteins, and NAD, we observe the formation of SIR-nucleosome filaments with a diameter of less than 20 nm. Together, these observations suggest that the SIR complex associates with an extended chromatin fiber through interactions with two different regions in the nucleosome.

The Escherichia coli RNA degradosome is a multicomponent ribonucleolytic complex consisting of three major proteins that assemble on a scaffold provided by the C-terminal region of the endonuclease, RNase E. Using an E. coli two-hybrid system, together with BIAcore apparatus, we investigated the ability of three proteins, polynucleotide phosphorylase (PNPase), RhlB RNA helicase, and enolase, a glycolytic protein, to interact physically and functionally independently of RNase E. Here we report that Rh1B can physically bind to PNPase, both in vitro and in vivo, and can also form homodimers with itself. However, binding of RhlB or PNPase to enolase was not detected under the same conditions. BIAcore analysis revealed real-time, direct binding for bimolecular interactions between Rh1B units and for the RhlB interaction with PNPase. Furthermore, in the absence of RNase E, purified RhlB can carry out ATP-dependent unwinding of double-stranded RNA and consequently modulate degradation of double-stranded RNA together with the exonuclease activity of PNPase. These results provide evidence for the first time that both functional and physical interactions of individual degradosome protein components can occur in the absence of RNase E and raise the prospect that the RNase E-independent complexes of RhlB RNA helicase and PNPase, detected in vivo, may constitute mini-machines that assist in the degradation of duplex RNA in structures physically distinct from multicomponent RNA degradosomes.RNA metabolism is a complex process affecting the control of gene expression. In bacteria, a multicomponent ribonucleolytic complex termed the RNA degradosome (1-4) has been identified as playing an important role in the control of mRNA degradation (for recent reviews, see Refs. 5-11). The multicomponent complex consists of: the RNA endonuclease RNase E, whose activity is essential for Escherichia coli cell growth (12-14), RNA processing (15, 16), and degradation (17, 18); the 3Ј-5Ј exoribonuclease PNPase (19); RhlB RNA helicase (20); and enolase (21), an enzyme involved in the glycolytic pathway and other chaperonin proteins (3,22). Interestingly, in addition to mRNAs, highly structured, stable RNA fragments have also been found to be associated with RNA degradosome complexes (3, 23), which implies quality control by the RNA degradosome for the biogenesis of stable RNAs. Degradosome complexity and its cooperation with individual protein components acting on degradation-targeted RNA, in vivo, remains to be discovered.Various approaches revealing protein-protein interactions in the degradosome indicate that the C-terminal region of RNase E serves as a scaffold that directly binds PNPase, RhlB RNA helicase, and enolase (24,25). No other interactions among these component proteins have been detected (25) or reported. Recently, a mini-degradosome complex (26) containing the scaffold region (without the N-terminal enzymatic region of RNase E), RhlB RNA helicase, and PNPase was reconstituted in vitro. These experiments revealed a functional inte...

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Hutchinson-Gilford progeria syndrome (HGPS) is a human progeroid disease caused by a point mutation on the LMNA gene. We reported previously that the accumulation of the nuclear envelope protein SUN1 contributes to HGPS nuclear aberrancies. However, the mechanism by which interactions between mutant lamin A (also known as progerin or LAD50) and SUN1 produce HGPS cellular phenotypes requires further elucidation. Using light and electron microscopy, this study demonstrated that SUN1 contributes to progerin-elicited structural changes in the nuclear envelope and the endoplasmic reticulum (ER) network. We further identified two domains through which full-length lamin A associates with SUN1, and determined that the farnesylated cysteine within the CaaX motif of lamin A has a stronger affinity for SUN1 than does the lamin A region containing amino acids 607 to 656. Farnesylation of progerin enhanced its interaction with SUN1 and reduced SUN1 mobility, thereby promoting the aberrant recruitment of progerin to the ER membrane during postmitotic assembly of the nuclear envelope, resulting in the accumulation of SUN1 over consecutive cellular divisions. These results indicate that the dysregulated interaction of SUN1 and progerin in the ER during nuclear envelope reformation determines the progression of HGPS.

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RNase E isolated from Escherichia coli is contained in a multicomponent ''degradosome'' complex with other proteins implicated in RNA decay. Earlier work has shown that the C-terminal region of RNase E is a scaffold for the binding of degradosome components and has identified specific RNase E segments necessary for its interaction with polynucleotide phosphorylase (PNPase), RhlB RNA helicase, and enolase. Here, we report electron microscopy studies that use immunogold labeling and freeze-fracture methods to show that degradosomes exist in vivo in E. coli as multicomponent structures that associate with the cytoplasmic membrane via the N-terminal region of RNase E. Whereas PNPase and enolase are present in E. coli in large excess relative to RNase E and therefore are detected in cells largely as molecules unlinked to the RNase E scaffold, immunogold labeling and biochemical analyses show that helicase is present in approximately equimolar amounts to RNase E at all cell growth stages. Our findings, which establish the existence and cellular location of RNase E-based degradosomes in vivo in E. coli, also suggest that RNA processing and decay may occur at specific sites within cells.immunogold labeling ͉ RhlB RNA helicase ͉ RNA process ͉ RNA degradation R Nase E is an essential Escherichia coli ribonuclease that has a key role in the degradation and͞or processing of both short and long half-lived RNAs. When purified from E. coli cells, RNase E is present in a multicomponent ribonucleolytic complex (i.e., the RNA ''degradosome'') that includes polynucleotide phosphorylase (PNPase), the RhlB RNA helicase, enolase, the DnaK chaperonin protein, GroEL, and polynucleotide phosphate kinase (PPK) (1-5). Specific regions required to bind certain degradosome proteins have been identified within the C-terminal half of RNase E (6), and a functionally active minimal degradosome containing only RNase E, PNPase, and helicase has been reconstituted in vitro (7).The view that many, if not most, cellular functions are carried out in vivo by multicomponent macromolecular complexes (i.e., cellular machines) rather than by individual freely diffusable proteins has gained wide acceptance in recent years (8). Well recognized and extensively studied examples of such complexes in bacteria and higher organisms include ribosomes, replisomes, and proteasomes (9-11). However, notwithstanding the isolation of multicomponent RNase E-based complexes from E. coli (1-5, 12), there has been no direct evidence that degradosomes are present in living cells-rather than being formed in vitro as aggregates of individual proteins. The question of whether degradosomes actually exist in vivo in E. coli is especially relevant in view of evidence that truncated RNase E protein lacking the C-terminal half is sufficient for cell viability and for RNA degradation and processing in vivo in E. coli (13,14), that RNase E homologs in certain other bacteria do not contain the scaffold region that interacts with PNPase and other degradosome proteins (15), and that puri...

Background: Salt-inducible kinase (SIK) 2 is an AMP-activated protein kinase family kinase that mediates hormonal and nutrient signaling but has no known link to cellular stress response. Results: p300/CBP and HDAC6 reciprocally regulates Lys-53 acetylation of SIK2, consequently impacting its activity and function in autophagosome maturation. Conclusion: SIK2 kinase activity, via a acetylation-based regulatory switch, contributes to autophagy progression. Significance: SIK2 may be linked to neurodegenerative or protein aggregate disorders.