Abstract

The filamentous fungus Neurospora crassa is made up of interconnected cells where nuclei and other cellular components share a common cytoplasm.  This trait, while beneficial for distributing resources, may promote the spread of detrimental elements such as transposons and viruses.  Perhaps for this reason, Neurospora possesses several surveillance mechanisms that operate during different phases of its lifecycle.  One of these defense mechanisms is known as meiotic silencing by unpaired DNA (MSUD).  In MSUD, genes not paired during meiosis are targeted by a post-transcriptional gene silencing pathway.  Here, our bimolecular fluorescence complementation (BiFC) study suggests that common RNAi proteins (RNA-directed RNA polymerase, Dicer, and Argonaute) as well as others form a meiotic silencing complex in the perinuclear region (the “surveillance checkpoint”), with intimate interactions among the majority of them.  We have also shown that SAD-2 (a putative scaffold protein) is likely the anchor for this assembly.

Complex formation of RNA silencing proteins in the perinuclear region of Neurospora crassa

Logan M. Decker,* Erin C. Boone,* Hua Xiao,* Benjamin S. Shanker,* Shannon F. Boone,* Shanika L. Kingston,† Seung A. Lee,* Thomas M. Hammond,‡ and Patrick K.T. Shiu**Division of Biological Sciences, University of Missouri, Columbia, MO 65211, †Department of Biology, Barry University, Miami Shores, FL 33161, ‡School of Biological Sciences, Illinois State University, Normal, Illinois, 61790.

Acknowledgements

We thank James Birchler and Patrice Albert for their sharing of equipment and advice. We are indebted to Tony Perdue, Patricia Pukkila, the Fungal Genetics Stock Center, the Neurospora Functional Genomics Group, University of Missouri Core Facilities, colleagues from our community, and members of the Shiu laboratory for their indispensible help. We are pleased to acknowledge use of materials generated by P01 GM068087 “Functional Analysis of a Model Filamentous Fungus”. L.M.D. was supported by a GK-12 Fellowship from the National Science Foundation (DGE1045322). E.C.B. was supported by a Graduate Assistance in Areas of National Need Fellowship from the U.S. Department of Education. T.M.H. was supported by a Life Science Fellowship from the University of Missouri and a Ruth L. Kirschstein National Research Service Award from the National Institute of General Medical Sciences. This work was supported by the National Science Foundation (MCB1157942 to P.K.T.S.).

AAA

+ SAD-2 − SAD-2

SAD-1-GFP

SAD-3-GFP

GFP-DCL-1

QIP-GFP

GFP-SMS-2

+ SAD-3 − SAD-3

SAD-1-GFP

SAD-2-GFP

Figure 2. SAD-2 is required for the localization of other perinuclear MSUD

proteins

(A) Exported aberrant RNA (blue) is turned into double strands by SAD-1 RdRP and SAD-3 helicase. (B) DCL-1 Dicer processes the dsRNA into siRNAs, (C) which (with the help of QIP exonuclease) become single-

stranded (green) and (D) are utilized by SMS-2 Argonaute to target homologous mRNAs (purple). SAD-2, a presumptive scaffold protein,

holds these silencing factors in the perinuclear region.

SAD-2(scaffold)

SAD-1(RdRp)

SAD-3(Helicase)

SMS-2(Argonaute)

QIP(Exonuclease)

DCL-1(Dicer)

A. dsRNA

B. siRNAs(duplex)

C. siRNAs (single-stranded)

D. mRNAtargeting

Figure 3. A model for reactions performed by the meiotic silencing complex

• In the absence of SAD-2, other MSUD proteins lose their affinity for the nuclear periphery (A–J).

• As with SAD-16, SAD-3 does not appear to affect the localization of others.

Meiotic silencing by unpaired DNA

• In Neurospora, genes unpaired during meiosis are silenced by a genome surveillance mechanism known as meiotic silencing by unpaired DNA (MSUD)1-7

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colocalizes with other components of the MSUD machinery and is required for silencing. Fungal Genet. Biol. 45: 719-727.

2. Hammond T.M., Xiao H., Boone E.C., Decker L.M., Lee S.A., Perdue T.D., Pukkila P.J., and Shiu P.K.T. (2013) Novel proteins required for meiotic silencing by unpaired DNA and siRNA generation in Neurospora crassa. Genetics 194: 91-100.

3. Hammond T.M., Xiao H., Boone E.C., Perdue T.D., Pukkila P.J., and Shiu P.K.T. (2011) SAD-3, a putative helicase required for MSUD, interacts with other components of the silencing machinery. G3 1: 369-376.

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5. Shiu P.K.T., Raju N.B., Zickler D., and Metzenberg R.L. (2001) Meiotic silencing by unpaired DNA. Cell 107: 905-916.

6. Shiu P.K.T., Zickler D., Raju N.B., Ruprich-Robert G., and Metzenberg R.L. (2006) SAD-2 is required for Meiotic Silencing by Unpaired DNA and perinuclear localization of SAD-1 RNA-directed RNA polymerase. Proc. Natl. Acad. Sci. USA 103: 2243-2248.

7. Xiao H., Alexander W.G., Hammond T.M., Boone E.C., Perdue T.D., Pukkila P.J., and Shiu P.K.T. (2010) QIP, an exonuclease that converts duplex siRNA into single strands, is required for Meiotic Silencing by Unpaired DNA. Genetics 186: 119-126 .

QIP

Nucleus

• Our results, as shown in Figure 1, indicate close association among various MSUD proteins in the perinuclear region.

• One noticeable exception is DCL-1, which appears to show affinity for SAD-1, SAD-2, and SMS-2 only (Figure 3).

• The results in Figure 2 indicate that SAD-2 is crucial for the perinuclear localization of SAD-3, DCL-1, QIP, and SMS-2.

Results

Figure 1. Assay of interactions among MSUD proteins using BiFC

BiFC Technique

No interaction

Interaction