© 2006 Nature Publishing Group http://www.nature.com/nsmb

RESEARCH HIGHLIGHTS

Piwi pathway for retrotransposon silencing

Please release me

In the last few years, several pathways for
RNA-mediated gene silencing have been
defined in Drosophila melanogaster. The
Argonaute (AGO) proteins AGO1 andAGO2, expressed ubiquitously throughout development, are essential in two ofthe pathways, with AGO1 binding microRNAs in one and AGO2 binding small interfering RNAs (siRNAs) inthe other. Both activities lead to repression of transcripts complementary to the bound RNAs. The functions of other AGO-related proteins
have remained relatively obscure, although mutants of Piwi, an AGO
relative expressed primarily in germline stem cells, are known to affectthe expression of retrotransposons. A new study by Siomi and colleagues further elucidates a silencing pathway involving specific interaction of Piwi with repeat-associated siRNAs (rasiRNAs) complementaryto retrotransposons. The authors identified rasiRNAs bound to Piwi
(but not to AGO1), whereas no miRNAs coimmunoprecipitated andsiRNAs were not loaded onto Piwi in cell lysates. Unlike AGO1 and AGO2,Drosophila Piwi localizes in the nucleus; it also lacks the Asp-Asp-His
catalytic motif shared by AGO1 and AGO2, yet has a similar ability tocleave target transcripts in vitro. The Piwi-bound rasiRNAs match transposable transcripts in both the sense and antisense directions. Chemicalend groups on these RNAs imply that they are products of RNase III
activity similar to that of the Dicer enzymes, which generate siRNAsand microRNAs and load them onto AGO proteins. As no such loading protein was found in Piwi immunoprecipitates, the factors andmechanisms that underlie rasiRNA processing remain to be determined.
(Genes Dev. 20, 2214–2220, 2006)
AB

During the process of translation, eukaryotic elongation factor eEF1abrings an amino-acyl transfer RNA to the acceptor (A)-site of the ribosome. Once the peptidyl transferase reaction takes place, eEF2 catalyzes
the movement of messenger RNA and tRNA from the A- and peptidyl(P)-sites to the P- and exit (E)-sites. eEF3 is an ATP-binding cassette(ABC) protein that converts chemical energy from ATP binding orhydrolysis into a ‘power stroke’ of mechanical energy. It was not clear
how eEF3 interaction with the ribosome was correlated with its dynamic
properties as an ABC protein and how the energy from binding orhydrolyis of ATP was used for its function in tRNA release. The crystal
and cryo-EM structures of eEF3, determined by Beckmann and coworkers, begins to address these questions. The protein is organized
into five structural domains: an N-terminal HEAT repeat, a four-helix
bundle, two ABC-type ATPase domains and a chromodomain inserted
within the second ABC domain. The cryo-EM structure of ATP-boundeEF3 is complexed with a post-translational ribosome with a peptidyltRNA in the P-site. In this complex, the eEF3 chromodomain lies near
the ribosomal E-site, where it may interact with ribosomal regions
which influence the affinity of tRNA for the E-site. This new information, combined with knowledge gained from previous studies, suggeststhat a conformational switch in the eEF3 chromodomain upon binding to the ribosome could stabilize an open conformation that allows
tRNA release. (Nature, advance online publication 23 August 2006,
doi:10.1038/nature05126)
BKGiving G1 feedback
In yeast, two transcription factor complexes, SBF and MBF, affect the expression of G1-specific genes. How MBF regulates transcription is unclear.
Some studies in budding yeast have suggested that MBF, a heterodimerof Swi6 and Mbp1, might repress rather than facilitate transcription ofits target genes, but other studies in fission yeast have found that the MBF
complex might also be involved in G1-specific transcriptional activation.
A recent study by Wittenberg and colleagues provides insight into MBFmediated regulation by identifying a new MBF-interacting factor, Nrm1,in budding yeast. Nrm1 localizes to the MBF-bound promoters of genes
repressed outside of G1. Deletion of NRM1 disrupts the cell’s ability torepress G1-specific genes upon G1 exit. One of these genes is NRM1 itself,
which is expressed late in G1. When the MBP1 gene is mutated, transcription of NRM1 (as well as other MBF targets) increases in late G1 and isprolonged. SBF is responsible for repression of NRM1 in early G1, but,surprisingly, maximal transcription of NRM1 in late G1 also requires SBF.
Nrm1 protein stabilization reduces G1 transcription of MBF-dependent,
G1-specific genes, confirming that its function is primarily repressive.
Nrm1 is also present in fission yeast MBF, where it also acts with MBF
to repress the transcription of G1-specific genes in late G1, as cells aretransitioning into S phase. These studies reveal a Nrm1 negative feedback loop that, together with MBF, controls the downregulation ofgenes expressed in G1. However, what counteracts MBF’s repression
during G1 to allow these genes to be expressed remains unknown.
(Mol. Cell 23, 483–496, 2006)
AKE

Research highlights written by Anna Barnett, Angela K. Eggleston,
Boyana Konforti and Michelle Montoya.

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Kip3p is a plus
The kinesin superfamilies are important regulators of microtubuledynamics. Most kinesins are motor proteins that transport
microtubule-binding proteins to microtubule tips, promoting
microtubule stability. Kinesins 13 and 8, however, have been
implicated in microtubule destabilization. Kinesin-13 lacks motor
activity but behaves as a microtubule depolymerase, actively
removing tubulin from microtubule ends. Although kinesin-8
is closely related to kinesin-13, it is unclear whether it shares
similar depolymerase activity. Kinesin-8 defects or deletions
lead to longer cytoplasmic microtubules and mitotic spindle
mispositioning, suggesting significant roles in cytoskeletal
dynamics. Two related papers from the Pellman and Howard
laboratories help clarify these roles. Using budding yeast kinesin-8(Kip3p), these groups show that the protein has both plus
end–directed motor activity and plus end–specific depolymerase
activity, making it unique in the kinesin families. Data fromthe Howard group show length-dependent Kip3p depolymerase
activity, with maximal depolymerase activity targeted to the plus
ends of longer microtubules by the highly processive plus end–
directed Kip3p kinesin motor. This activity may help maintainaverage cellular microtubule length. The Pellman group foundthat, similar to kinesin-13, Kip3p has tubulin dimer–dependent
ATPase activity. Analysis of kip3∆ cells indicates that Kip3p
maintains appropriate microtubule lengths needed for spindle
positioning at the bud neck during preanaphase. Together, thesestudies suggest roles for Kip3p depolymerization activity that may
be common to all kinesin-8 proteins. It will be interesting to see
how the depolymerase and motor activities are involved in theseveral other microtubule functions in which kinesin-8 has been
implicated. (Nat. Cell Biol., advance online publication 13 August
2006, doi:10.1038/ncb1457 and doi:10.1038/ncb1462) MMVOLUME 13 NUMBER 9 SEPTEMBER 2006 NATURE STRUCTURAL & MOLECULAR BIOLOGY