Function of the Ski4p (Csl4p) and Ski7p Proteins in 3’-to-5’ Degradation of mRNA

Ambro van Hoof, Robin R. Staples, Richard E. Baker, and Roy Parker

Molcecular and Cellular Biology, p8230-8243 (2000)

Yeast virus

Introduction

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Cryo-EM 16Å Crystal structure 3.4Å

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The viral replication cycles of L-A and its satellites

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SKI antiviral system

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A60-80mRNA7mGppp

mRNA degradation in yeast

A0-10mRNA7mGpppA60-80mRNA p

DeadenylationDecapping (Dcp1p, Dcp2p)

A60-80mRNA

5’-3’ degradation (Xrn1p)

mRNA7mGppp

3’-5’ degradation

The exosome: a conserved eukaryotic RNA processing complex containing multiple 3’-5’ exoribonuclease activities. Cell, 91, 457-466 (1997)

Rrp4p,Rrp41/Ski6p, Rrp42p, Rrp43p, Rrp44p

The 3’ to 5’ degradation of yeast mRNAs is a general mechanism for mRNA turnover that requires the SKI2 DEVH box protein and 3’ to 5’ exonucleases of the exosome complex EMBO J. 1497-1506 (1998)

The SKI2, SKI3, SKI6/RRP41, SKI8 and RRP4 gene products are required for 3’ to 5’ decay of mRNA

The yeast antiviral proteins Ski2p, Ski3p, and Ski8p exist as a complex in vivoRNA 6:449-457 (2000)

The ski4-1 and ski7Δ mutations affect the metabolism of a degradation intermediate of MFA2pG mRNA

The ski4-1 and ski7Δ mutations stabilize a degradation intermediate of MFA2pG mRNA

Strains containing either ski4-1 or ski7Δ in combination with dcp1-2 or dcp2-7 are not able to grow under conditions restrictive for the decapping defect

SKi4p and Ski7p proteins are required for 3’-5’ mRNA degradation as are the components of the exosome and the Ski2p,SKi3p, and Ski8p protein.

The ski4-1 mutation is complemented by a wild-type (WT) CSl4 gene

A mutation in the conserved S1 RNA binding domain of Csl4p is responsible for the ski4-1 phenotype

Two mutations of Ski4p : R65K, G253E

The SKI4 gene is identical to the CSl4 gene

The ski4-1 mutation does not affect all Csl4p functions

ski6

The csl4-1 mutation affects rRNA processing but not mRNA degradation

rRNA processing

mRNA degradation

The ski4-1 allele genetically separates the functions of the exosome in rRNA processing and mRNA decay

The ski7Δ and hgs1Δ mutations do not affect 5.8S rRNA or U4 snRNA processing or 5’ ETS degradation

The SKI7 gene is specifically required for mRNA degradation by the exosome

Summary

1. The Ski4p and Ski7p proteins are required for mRNA degradation.

2. The SKI4 gene is identical to the CSL4 gene

3. The ski4-1 allele genetically separates the functions of the exosome in rRNA processing and mRNA decay

4.The SKI7 gene is specifically required for mRNA degradation by the exosome.

SKI 1: Xrn1p enzyme (5’ to 3’ exoribonuclease)SKI 2: 1287 a.a. putative RNA helicaseSKI 3: 1432 a.a. tetratricopeptide repeat (TRP) proteinSKI 4: 195 a.a. S1 domain (Csl4p)SKI 6: 264 a.a. Rrp41p (3’-5’ exoribonuclease) SKI 7: 747 a.a. GTPase domain ( translation elongation factor EF1A/ translation termination factor eRF3)SKI 8: 397 a.a. five WD-40 (beta-transducin) repeats protein

Componenets of the exosome complex