The Chaperone-Like Protein HYPK Acts Together with NatA in Cotranslational N-Terminal Acetylation and Prevention of Huntingtin Aggregation

Abstract

The human NatA protein N^α-terminal-acetyltransferase complex is responsible for cotranslational N-terminal acetylation of proteins with Ser, Ala, Thr, Gly, and Val N termini. The NatA complex is composed of the catalytic subunit hNaa10p (hArd1) and the auxiliary subunit hNaa15p (hNat1/NATH). Using immunoprecipitation coupled with mass spectrometry, we identified endogenous HYPK, a Huntingtin (Htt)-interacting protein, as a novel stable interactor of NatA. HYPK has chaperone-like properties preventing Htt aggregation. HYPK, hNaa10p, and hNaa15p were associated with polysome fractions, indicating a function of HYPK associated with the NatA complex during protein translation. Knockdown of both hNAA10 and hNAA15 decreased HYPK protein levels, possibly indicating that NatA is required for the stability of HYPK. The biological importance of HYPK was evident from HYPK-knockdown HeLa cells displaying apoptosis and cell cycle arrest in the G₀/G₁ phase. Knockdown of HYPK or hNAA10 resulted in increased aggregation of an Htt-enhanced green fluorescent protein (Htt-EGFP) fusion with expanded polyglutamine stretches, suggesting that both HYPK and NatA prevent Htt aggregation. Furthermore, we demonstrated that HYPK is required for N-terminal acetylation of the known in vivo NatA substrate protein PCNP. Taken together, the data indicate that the physical interaction between HYPK and NatA seems to be of functional importance both for Htt aggregation and for N-terminal acetylation.

Supplemental material for this article may be found at http://mcb.asm.org/.

We thank N. Glomnes, K. Jacobsen, C. Hoff, and E. Skjelvik for technical assistance.

This work was supported by The Meltzer Foundation (grant to T.A.), the Western Norway Regional Health Authority (grant to T.A.), and the University of Bergen (research grant to T.A.). P.V.D. is a postdoctoral fellow at the Fonds voor Wetenschappelijk Onderzoek-Vlaanderen. The Department of Medical Protein Research further acknowledges support by research grants from the Fund for Scientific Research-Flanders (Belgium), the Concerted Research Actions from Ghent University, the Interuniversity Attraction Poles, and the European Union Interaction Proteome (6th Framework Program).